Meteorology Recurring Phenomena
pressure winds north temperature south rainfall region regions inches west
METEOROLOGY RECURRING PHENOMENA, The Temperature of the Sea. - Figs. 8 and 9, representing the distribution of the temperature of the surface water of the ocean for the two extreme months February and August, are reproduced chiefly from The Wind and Current Charts for Pacific, Atlantic, and Indian Oceans, published by the British Admiralty in 1872.
In February (fig. 8) the temperature of the surface of the sea falls to the annual minimum over the northern hemisphere, and rises to the maximum in the southern hemisphere. The course of the isothermals more closely follows the latitudes in the Pacific, Indian, and South Atlantic Oceans ; but the divergence from the latitudes is great and striking over the North Atlantic. The wider and more open the ocean the more does the distribution of the temperature approach the normal; and the more confined the ocean the greater is the divergence from the normal. The key to the anomalous distribution of the temperature of the ocean is furnished by the charts of the distribution of atmospheric pressure and the prevailing winds of the globe. So far as observation has gone it would appear that the surface currents are practically altogether caused by the prevailing winds over the respective oceans, subject to such deflexions in their courses as are occasioned by the land.
In the southern hemisphere the currents on the west side of the Indian Ocean flow southwards along the east coast of Africa, and, since the currents here pass from lower to higher latitudes, the temperature along the whole extent of this coast is raised considerably above the normal. On the other hand, since the currents on the west coast of Africa flow from south to north - in other words, from higher to lower latitudes - the ocean currents which impinge on this coast have a temperature much under the normal. The winds and currents on the coasts of South America are precisely analogous to those of Africa, and the distribution of the temperature of the sea is also similar. The temperature of the ocean on the east eoast of that continent is for the same latitudes everywhere higher than on the west coast. Even in the smaller continent of Australia the same law holds good.
In the northern hemisphere a different distribution of the temperature of the sea is seen at this season. In the Atlantic the temperature is very much higher on the west of Europe than on the east of America. On the east of America from Wilmington to Boston occur the most rapid transitions in the mean temperature of the ocean anywhere on time globe, the temperature falling in that short distance from 70° to 30°, whereas on the eastern side of the Atlantic these isothermals pass Cape Yerd Islands and Spitzbergen respectively. In the winter months the prevailing winds of the east side of North America are north-westerly, whilst i-n the central and eastern portion of the Atlantic they are south-westerly, thus pouring along the east coast of America the icy currents of the Arctic regions, but over the central Atlantic and along the western shores of Europe the warm waters of southern climates. The easterly and south-easterly winds of Scandinavia in winter lower the isothermals along these coasts. A striking feature of the winter isothermals of the Atlantic is the singularly high temperature along the centre stretching from Spitzbergeu towards the south-west and extending in a modified degree as far south as the West Indies. In the Pacific this feature of the mid-ocean temperature is much less pronounced, and the excess of temperature on the west of America over what occurs in the same latitudes of eastern Asia is not so great as the difference observable between the two sides of the Atlantic.
The highest mean temperature in February (85°) occurs in the Indian Ocean to the south-west of Sumatra, and there is a patch the temperature of which is 84° to the north of Madagascar. The highest means in the Atlantic are 82° in the north-east angle of the Gulf of Guinea, and 81° off the north-east coast of Brazil. In the Pacific the highest are 83° to the north of the Fiji Islands and 81° near the Marshall Islands.
In August (fig. 9) the southern half of the Red Sea shows a mean temperature of 90°, being the highest mean recorded for the ocean anywhere at any season. Patches showing a summer mean of 85° occur in the Chinese Sea to the east of Tonquin, in the Bay of Bengal to the east of southern India, about Socotra, and to the west of Central America. But the most extensive regions of high temperature are in the west of the Pacific between long. 165° E. and the Philippines northward nearly to Japan and southward to New Guinea, and the Gulf of Mexico and the adjoining part of the Atlantic as far east as long. 57° W. A patch of remarkably low temperature occurs iu the Pacific a little to the west of Galapagos, where the mean is only 70°, being 10° lower than what occurs anywhere else near the equator at this season.
The influence of currents is strongly expressed in the temperature of all the oceans. In the south of Asia the monsoons are S.W., S., and S. E. Under the impulse of these monsoonal winds an extensive surface drift of the waters of the equatorial regions is carried northwards towards southern Asia, and consequently very high temperatures characterize these seas in summer. It is instructive to note the effect on the temperature of the sea resulting from the region of high atmospheric pressure in the North Atlantic at this season. Out of this anticyclonic region the winds blow in all directions, giving rise to surface currents flowing in the same directions. Thus to the west of Africa the winds and currents are from north to south ; and hence the temperature of this part of the ocean is abnormally reduced. On the other hand, on the west side of this high pressure area, the prevailing winds and currents are from south to north, and it will be seen that the temperature of the whole of the region swept by the southerly winds is abnormally raised. On the north side of the area, the winds and currents are westerly as far as about long. 35° W., and over that space the isothermals follow the parallels of latitude. Farther to eastward and northward the prevailing winds become south-westerly, thus propelling northwards along the western shores of Europe, by oceanic surface drifts, the warmer waters of southern latitudes. Meanwhile the currents of cold water and ice drifts from the Arctic regions keep the temperature off America to the north of Newfoundland at a figure considerably lower than is observed in any other region in the same latitudes. In August similar relations exist as in January between the east and west coasts respectively of South Africa, South America, and Australia, all of which are readily explained by the charts of mean atmospheric pressure and the resulting prevalent winds.
One of the most striking facts of ocean temperature is that the temperature of the Southern Ocean from about 50° to 60° S. lat. is practically the same in January and August, a circumstance due chiefly to the magnificent icebergs of that ocean.
The Temperature of the Land. - In regions where the rainfall is distributed through all the months of the year, and where snow covers the ground for only a small part of the year, the mean temperature of the soil nearly equals that of the air. But when the year is divided into wet and dry seasons, and when snow lies during a considerable portion of the year, the mean annual temperature of the soil may be above or below that of the air. The greatest difference between the temperature of the soil and that of the air occurs where the surface of the ground is covered during several months with snow. Snow is a bad conductor of heat, and thus obstructs the free propagation of the cold produced by radiation downwards into the soil, and the escape of heat from the soil into the air. In this way, over a considerable portion of the Russian empire, the temperature of the soil is considerably in excess of that of the air. Thus at a place 120 miles south of Archangel the temperature of the soil is 10° higher than that of the air ; and at Semipalatinsk it is 9° higher.
The daily changes of temperature only affect the soil to depths of about 4 feet. The precise depth varies with the degree of the sun-heat and with the nature of the soil. Similarly the heat of summer and the cold of winter give rise to a larger annual wave of heat propagated downwards, the amplitude of which diminishes with the depth till it ceases to be perceptible. Principal Forbes showed from observations on the Calton Hill, Edinburgh, that the annual variation is not appreciable lower than 40 feet below the surface, and that under 25 feet the change of temperature through the year is small. The depth at which the annual variation ceases, or where the temperature remains constant, is a variable depending on the conductivity and specific heat of the soil or rock, but particularly on the difference between the summer and winter temperatures. The rate at which the annual wave of temperature is propagated downwards is so slow that at Edinburgh, at a depth of 24 feet, the highest annual temperature does not occur till January 4, and the lowest till about July 13, thus reversing the seasons at this depth. At Greenwich, at a depth of 25i feet, these phases of the annual temperature occur on November 30 and June 1.
Professor Everett in the Report of the British Association for 1879 has summarized the results of the observations of underground temperature. The temperature of the surface of the ground is not sensibly influenced by the flow of heat from below upwards, but is determined by atmospheric and astronomical conditions. The tem perature gradient is defined as the rate of increase of the temperature downwards, and it may be taken as averaging one degree Fahrenheit for every 50 or 60 feet, the exact rate in particular cases being very variable. Thus the temperature gradient of the soil is about five times steeper than the temperature gradient of the air. The temperature gradient is steepest beneath gorges and least steep beneath ridges ; and hence the underground annual isothermals are flatter than the uneven surfaces above them. This is the ease even with the uppermost isothermal of the soil, and the flattening increases as we pass downwards until at a considerable depth they become horizontal. Where the surface of the ground and the isothermal surfaces beneath it are horizontal, the flow of heat is vertical, and the same quantity of heat flows across all sections which lie in the same vertical. In this case the flow across a horizontal area of unit size is equal to the product of the temperature gradient by the conductivity, if the latter term be used in an extended sense, so that it includes convection by the percolation of water, as well as conduction proper ; and hence, in comparing different strata in the same vertical, the gradient varies in the inverse ratio of the conductivity.
Since the effects of the cold generated by nocturnal radiation mostly accumulate on the surface of the earth, but the effects of solar radiation are spread to some height by ascending currents from the heated ground, it might be expected that the annual temperature of the surface layer of the soil would be lower than that of the air resting over them. Observations prove that such is the ease. 'Springs which have their sources at greater depths than that to which the annual variation penetrates have a constant temperature throughout the year, and if they do come from a depth considerably greater than this they may be regarded as giving a very close approximation to the mean annual temperature of the place. The temperature of cellars is also very near the mean annual temperature of the locality; at any rate this temperature may be secured for cellars anywhere.
Distribution of Temperature in the Atmosphere. - Of the larger problems of meteorology, the distribution of temperature in the atmosphere over the land surfaces of the globe was the first that received an approximate solution (by Humboldt). But as regards the ocean, which comprises three-fourths of the earth's surface, the question of the monthly and annual distribution of temperature in the atmosphere over it can scarcely yet be said to have been seriously looked at. The isothermals of the temperature of the atmosphere which cross the oceans continue still to be drawn essentially from observations made on the islands and along the coasts of these oceans. The first step towards the solution of this vital problem in climatology and other branches of meteorology is the construction of charts of mean monthly temperature of the surface water of the sea over all parts of the ocean from which observations for the purpose are available. In prosecuting this line of inquiry, excellent work has been done by the Meteorological Office as regards parts of the Atlantic between the tropics and the ocean to the south of Africa, and also by the Dutch, French, and German meteorologists. With such charts it would not be difficult, by a careful comparison during the same intervals of time between the temperature of the surface of the sea and that of the air resting over it, to construct monthly charts of the temperature of the atmosphere over the oceans of the globe.
In this connexion the whole of the observations of the temperatures of the air and sea made on board the " Challenger " have been examined, and sorted into one hundred and seventy-four groups according to geographical position, and the differences entered on a chart of the route of the expedition. In the Southern Ocean between latitudes 45° and 60° the temperature of the sea was lower than that of the air, the mean difference being 1'4. The temperature of the air is here higher owing to the prevailing W. N. W. winds, and that of the sea lower owing to the numerous icebergs. To south of lat. 60° S. the sea was nearly 20.0 warmer than the air, the result in this case being due to the open sea, which keeps up a higher surface temperature, and to an increased prevalence in these higher latitudes of southerly winds, thus lowering the temperature of the air.
The period during which the temperature of the sea exceeded that of the air was from June 1874 to March 1875, or during that part of the cruise from Sydney to New Zealand, and through the East India Islands to Hong Kong and thence to the Admiralty Islands. During the whole of this time, except when passing the north of Australia, the sea was much warmer than the air, the general excess being from 2° to 3°, rising even near Tongatabu to • upwards of 4°. The climate of the southern part of this extensive region at the seasons visited has a large rainfall, much cloud, and • consequently a comparatively small evaporation and sunshine. In June, when the "Challenger" passed the north of Australia, the climate was very dry, the sunshine strong, and the evaporation large, and there the sea was slightly colder than the air. In the Atlantic between lat. 20° N. and 20° S. the sea was everywhere warmer, the mean excess being about a degree; and in the Pacific between lat. 30° N. and 30° S. the sea was also warmer, the mean excess being a degree and a half.
On the other hand, in the Atlantic from lat. 40° to 20° N. the sea was, on the mean, half a degree colder than the air. This region is remarkable for the high pressure which overspreads it, for the winds and currents which flow out in all directions, for its clear skies, strong sunshine, and consequently large evaporation, by which the temperature of the surface of the sea is lowered, and that of the air resting on it, being open to the heating influence of the sun, is raised. Similarly in the North Pacific from lat. 40° to 30° the temperature of the surface of the sea was half a degree lower than that of the air.
These remarks apply only to the observations made strictly on the open sea. Near land very great differences were observed which varied with season. Thus at IIong Kong during the latter half of November 1874 the sea was 3°.7 warmer than the air, the low temperature of the air at this season being caused by the lower temperature of the land and the northerly winds which then prevail; on the other hand, at Valparaiso in November and December of the following year the sea was 5°•8 colder than the air during the three weeks the "Challenger" was there, the difference being due to the cold oceanic current which sweeps northwards past that coast, and the rapid increase in the temperature of the air at that time of the year. These results will help us in gaining some knowledge of the temperature of the air over the oceans of the globe in February and August, taken in connexion with a careful examination of the sea temperature of these months represented in figs. 8 and 0.
The distribution of temperature over the surface of the globe is shown by figs. 10 and 11, which represent the temperature of the two extreme months January and July for the eleven years 1870 to 1880. The region of highest temperature, which may be regarded as comprised between the north and south isothermals of 80°, forms an irregularly shaped zone, lying in tropical and partly in subtropical countries. On each side of this warm zone the temperature diminishes towards the poles, and the lines showing successively the gradual lowering of the temperature are, roughly speaking, arranged parallel to the equator, thus showing in an unmistakable manner the predominating influence of the sun as the source of terrestrial heat. While this decrease of temperature corresponds in a general way to what may be called the solar climate, there are great deviations brought about by disturbing causes.
Among these disturbing causes the unequal distribution of land and water holds a prominent place. In January the earth presents to the perpendicular rays of the sun the most uniform surface, or the largest water surface, and in July the most diversified surface, or the greatest extent of land. Hence the zone of the earth's surface comprised between the isothermals of 80° is less irregular, and also spreads over an area more restricted, in January than in July. In July the areas enclosed by the isothermals of L0° and 90° are much larger in the Old World than in the New, it being the former which presents the larger land surface to the perpendicular rays of the sun ; and in January, the slimmer Of the southern hemisphere, the most extensive area of high temperature occurs in Africa and the least in Australia, the high-temperature area of South America being intermediate, In contrast to this the belt of temperature exceeding 80° is of least breadth where it crosses the Pacific and Atlantic Oceans, the absolute minimum breadth being in July in the Pacific, the largest ocean, where the disturbing influence of the land is least.
During the cold month of the year, when the sun's boat is least and the effects of terrestrial radiation attain the maximum, the greatest cold is over the largest land surfaces which slant most to the sun. Thus the lowest mean temperature that occurs anywhere or at any season on the globe is - 55°.8 at Werchojansk (lat. 07° 34' N., long. 133° 51' E.) in north-eastern Siberia. In Arctic America the lowest isothermal is - 40'0. During the winter the ocean everywhere maintains a higher temperature in all regions open to its influence, as is seen, not only in the higher latitudes to which the isothermals push their way as they cross the Atlantic and Pacific, but also in their irregular courses over and near the Mediterranean, Black, Caspian, and Baltic Seas, Hudson's Bay, the mouth of the St Lawrence, the American lakes, and all other large sheets of salt and fresh water. The disturbing influence of sheets of water on the temperature in all seasons is very strikingly shown when the isothermals are drawn for every east of Australia and in the basin of the La Plata, a lower temperature prevails in the interior.
Another prominent disturbing cause operating on the 1,,w pressure about Iceland during the winter months (see fig. 14). Since this region of low pressure gives to western Europe its prevailing south-west and south winds, and to North America its north-west winds in winter, it is plain that the temperature of western Europe is thereby abnormally raised by the simple fact of its prevailing winds coining from the ocean and from lower latitudes, and that the temperature of North America is abnormally lowered by its prevailing winds coming from the Arctic regions and from land. The opposite action of these two winds, which are part and parcel of the same atmospheric disturbance about Iceland, is shown from the fact that, while the mean temperature of the south coast of Hudson's Bay in January is - 20°, in the same latitude in the Atlantic to the west of Scotland it is as high as 44°, or 64° higher. A similar though less striking result accompanies the low-pressure area in the north of the Pacific in winter.
Another area of low mean pressure which powerfully affects the temperature is the low barometer which overspreads the interior of Asia during the summer months (see fig. 17). Since from this disposition of the pressure the prevailing winds of Europe and western Asia are northwest and west, and over eastern Asia south-east and east, it follows that the temperature is abnormally raised on the eastern side and depressed on the western side of the continent by the direction from which they severally receive their prevailing winds. This is well shown by the course of the summer isothermals of 80°, 70°, 60°, and 50° across the Old Continent.
Since the strongest insolation occurs where the air is driest, the hottest summer climates are met with in those tropical and subtropical regions where no rain falls. The most extensive of the rainless regions during the summer months is perhaps that which extends from the Punjab westwards through Persia, Arabia, and North Africa to Spain. This is the region where the hottest climates of the globe are to be encountered. Similarly no rain falls at this time of the year in lower California and the States adjoining, and this feature of the climate, taken in connexion with the relatively low temperature of the coast due to the winds and ocean currents from the north which sweep past it, results in sharp contrasts of temperature within short distances such as have no parallel in any other climate.
Of the areas of seasonal high mean pressure, the high barometer of Central Asia in winter stands out in characteristic prominence (see fig. 14). Now, since the prevailing winds which necessarily form a part of this feature are south and south-west over Russia and western Siberia, the temperature of these inland regions is considerably higher than would otherwise be the case. On the other hand, since the prevailing winds are north-west in eastern Asia, the temperature of these regions is thereby abnormally depressed. It is this consideration chiefly which explains how it is that, while the mean January temperature in latitude 60° and longitude 120° E. is - 30°, in the same latitude but in longitude 43° E. the mean temperature is 10°, or 40° higher, even though both regions are equally continental in their character.
The high mean pressure in the summer in the Atlantic between Africa and the United States has with its system of winds the most decided influence in bringing about the abnormal distribution of the temperature of that and adjoining regions. Since on its west side the prevailing winds are necessarily southerly, the temperature of that region is abnormally raised, and, on the other hand, since on its east side the winds are northerly, the temperature of the region is abnormally depressed. The result of these two opposite winds is seen in the slanting direction of the isothermal of 80° across the Atlantic, which slanting direction is continued far into the interior of North America for the reasons already stated.
These important bearings of cyclonic and anticyclonic areas on temperature and climate may be thus summarized. The temperature is abnormally raised on the east sides of cyclonic areas, and abnormally depressed on the west sides; but, on the other hand, temperature is abnormally raised on the west sides of anticyclonic areas and depressed on their east sides. In the southern hemisphere these directions are reversed.
Another set of influences, powerfully affecting the temperature, come into play where the surface of the land rises above the sea into elevated plateaus, lofty peaks, or mountain ranges. Thus it has been observed on Ben Nevis and other mountains that the wind during the day in summer exhibits an ascensional tendency due to the circumstance that the temperature of the surface of the mountain is heated in a much greater degree than the air strata at the same levels all around it. An ascensional current consequently rises from the mountain, which is maintained at a steadily stronger rate than at lower levels, because the drain from the updraught is easily supplied from the free surrounding atmosphere. It is the strong insolation at high elevations in the summer months which explains the excessively high day-temperatures encountered in the Rocky Mountains ; and from the same conditions, viz., the rarity and purity of the atmosphere, by which terrestrial radiation is but little checked, come the low temperatures of the nights of these climates in the same season. From this cause it follows that the elevated lands in the interior of continents tend to reduce mean atmospheric pressure in summer to a greater extent than would otherwise be the case. In winter, on the other hand, the temperature of elevated regions in the interior of continents is very much colder than that of the surrounding atmosphere at the same heights, because in such regions the air is exceedingly dry and rare, and consequently radiation to the cold regions of space but little checked. Hence down the slopes of these high lands there are poured in all directions descending currents of very cold air, which intensify the rigours of the winters experienced on the low lands round their base, where accordingly the lowest mean winter temperatures occur. These elevated lands thus materially add to the high atmospheric pressure of the interior of continents during the cold months of the year.
But it is ocean streams and ocean currents which produce the greatest abnormalities in the distribution of the temperature of the air, and a glance at figs. 10 and 11 will show that it is in the North Atlantic where this cause is most strikingly seen. The increase thus accruing to the winter temperature is greatest about the north of Norway. It is also very great in the British Islands ; thus, if no more heat were received than is due to their position on the globe in respect of latitude, the mean winter temperature of Shetland would be 3° and that of London 17°. But mainly owing to the heat given out by the Gulf Stream and other warm currents of the Atlantic their mean winter temperatures are respectively about 39'5 and 39°, Shetland being thus benefited 360.5 and London 22°. The chart of the winter temperature of the British Islands well illustrates the influence of the surrounding ocean in maintaining a higher temperature. It will be seen that the south-west of Ireland is 7° warmer than the east coast of England in the same latitudes. The strong drift current from near Behring's Strait southward along the coast of America has a powerful influence, particularly in lowering the summer temperature of that coast, - thus bringing about, in conjunction with the dry rainless climate of the interior, what are perhaps the most violently contrasted climates, within narrow limits, as regards their temperature. The deflexions of the isothermals near the Baltic, Mediterranean, Black, and Caspian Seas and the freshwater lakes of America all point to the disturbing influence of these sheets of water on the temperature.
The height and direction of mountain ranges is an important element in determining climate. If the ranges are perpendicular to the prevailing winds and of a considerable height, they drain the winds of much of their moisture, thus causing to places to leeward colder winters and hotter summers, by partially removing their protecting screen of vapour, and exposing them more completely to solar and terrestrial radiation. Of this Norway and Sweden and the British Islands form excellent illustrations. It is this that makes the most important distinctions among climates in regions near each other, as respects both animal and vegetable life. With regard to the decrease of temperature with height, very much yet remains to be done before an approximation to the law of decrease can be stated. During the five months observations were made on Ben Nevis in the summer of 1881 the difference between the mean temperature at sea-level adjoining and at the top of the Ben, 4406 feet above the sea, was 15•7, which shows a mean decrease of 1° Fahr. for every 280 feet of elevation. The actual differences from day to day varied from 1°•4 to 23°•2. As Ben Nevis forms a peak, and is in the very middle of the strong winds from the Atlantic, it is highly probable that this rate of decrease is a close approximation to the true decrease of the temperature of the air during the summer months in that part of the British Islands. When observations are made on elevated plateaus of some extent, the rate of decrease deduced from the observations will be less than the true rate in the free atmosphere in summer and greater in winter. The rate is thus a variable quantity, varying with latitude, situation, dampness or dryness of the air, calm or windy weather, and particularly with the season of the year. One degree Fahrenheit for every 300 is the rate of decrease generally assumed.
Amount of Aqueous Vapour. - It is scarcely possible to overestimate the importance of a knowledge of the horizontal and vertical distribution in the atmosphere of its aqueous vapour, for it may be truly said that it forms one of the prime factors in all the larger problems of atmospheric physics. A first rough approximation to the geographical distribution of the vapour of the atmosphere was published by Mohr" in 1875 in his Grundziige der Meteorologie, p. 84, in which vapour-pressure curves are drawn for the globe for January and July. These leave much still to be done, not only in a further discussion of observations already made, but also in improvement of the methods of observation and in the tables for their reduction. The chief point of interest in Mohn's vapour curves is their striking resemblance to the isothermals of the same months, and they also suggest that this line of inquiry is yet destined to make large contributions to our knowledge of the unceasing changes which occur in the pressure, temperature, cloud, rain, and movements of the atmosphere.
Still less is known of the vertical distribution of aqueous vapour. It decreases, like temperature, with the height, and if the statement generally made be at all correct, that half of the whole vapour of the atmosphere is contained in the lowest 6000 feet, and that at 20,000 feet high there is only about a tenth of what is at the earth's surface, the rate of decrease with height proceeds at a greatly more rapid rate than is consistent with the supposition that it forms an independent vapour atmosphere existing under its own pressure. The establishment of an increased number of high-level stations, and a more systematic inquiry than has yet been attempted into the upper currents of the atmo sphere, are much needed in the further development of this branch of meteorology. In carrying out the inquiry, invaluable assistance will be obtained from observations of the diurnal range of the barometer and from well-devised methods of observing the effects of solar radiation at the earth's surface.
Amount of Cloud. - In Scotland, which lies completely within the region swept by the south-westerly winds from the Atlantic, and presents a well-defined mountain range lying across the track of these winds, the clouds have a distinct annual period. In the west, at places quite open to these westerly breezes, the amounts of cloud in spring, summer, autumn, and winter are respectively 67, 69, 71, and 74, and the annual mean 70.1 In the east, in such districts as East and Mid Lothian, which have extensive ranges of hills between them and the Atlantic, the proportions are 59, 63, 62, and 60, and the annual mean 61. Thus about a tenth more of the sky is covered with cloud at the western as compared with the eastern situations, and the distribution of cloud differs materially in western and eastern climates. In the west winter is the cloudiest season, but in the east it is summer, and these are respectively the months when most rain falls in the several climates. Everywhere spring is the season when the sky is clearest. In England, owing to the protection afforded by Ireland and Wales to the west and the comparative absence of ranges of hills, the amount of cloud is less than in Scotland, and it is more equally distributed over the country. The minimum amount occurs in spring, and the maximum in winter and autumn.
Sonic of the best illustrations of the seasonal variation in the distribution of cloud are afforded by the Old Continent. These variations are the simple consequence of the systems of wind caused by the high winter and low summer pressures of that continent. In eastern Siberia the prevailing winds in whiter are N.W. or continental, and in summer S.E. or oceanic; and accordingly at Ajan, Nertchinsk, and Blagoweshtchensk the mean amounts of cloud in these two seasons are 18 and 44. On the other hand, in western Siberia and eastern Europe the prevailing winds in winter are S.W., or from lower to higher latitudes, and in summer N.W., or from higher to lower latitudes. Kazan may be taken as fairly representing this extensive region, and there the amounts of cloud for the four seasons beginning with winter are 71, 48, 44, and 62. As the N.W. winds of summer rise over the Ural mountains in their course, condensation of the aqueous vapour is increased, and hence over this region the cloud in winter and summer is nearly the same, the mean amounts at Bogoslovsk, Ekaterinburg, and Zlatoust being respectively 53 and 52. At Tiflis and Kutais, situated on the high ground which lies between the Black Sea and the south of the Caspian Sea, the means for winter and summer are 53 and 55. On the eastern coast of the Black Sea the westerly winds of summer are accompanied with the annual maximum cloud, the winter and summer amounts at Redut-Kale being 59 and 69. In Central Siberia, to which the S.W. winds of winter do not extend, and to the north of latitude 55°, the amount of cloud is much diminished, and the cloudiness of summer is nearly the same as that of winter.
In India, in all regions which lie open to the summer monsoon, the minimum amount of cloud occurs during the winter and the maximum in summer, - the mean amounts being 19 and 74 at Calcutta, 16 and 86 at Bombay, 48 and 71 at Colombo, and 25 and 90 at Rangoon. At Trincomalee, on the east coast of Ceylon, and thus exposed to the rains of the N.E. monsoon of winter, and largely protected from the rains of the SM. monsoon of summer, the amounts of cloud in these seasons are 52 and 59. At Darjiling (6912 feet) and Chakrata (7022 feet high), both on the Himalayas, whither the summer monsoon penetrates, the mean amounts are respectively 53 and 86, and 43 and 73. At Leh, in Kashmir, the amounts are 59 and 51, the excess being thus in winter. In the Punjab and to westwards, or those regions in southern Asia to which the summer monsoon does not extend, the cloud in winter is everywhere greater than in summer. Thus the amounts are 24 and 18 at Mooltan, 38 and 25 at Peshawar, 27 and 19 at Jacobabad, and at Quetta, in Baluchistan, 5500 feet high, 42 and 14. Similar relations as to cloud obtain in Australia and the other continents where high pressures rule in the interior during the cold mouths and low pressures during the warm months of the year. The maximum cloud occurs with winds from the sea and winds advancing into the colder regions of higher latitudes, and the minimum with winds which have traversed an extensive track of land and winds advancing into the warmer regions of lower latitudes. As the subject, however, is essentially one with rainfall, it is not necessary to prosecute it further.
The other atmospheric movements on which the amount of cloud depends are the ascending and descending currents of the atmosphere, - the ascending currents wi+1, clouded skies occurring in the belt of calms and over cyclonic areas and regions, and the descending currents with comparatively clear skies over anticyclonic regions. The region of maximum vapour and densest cloud-screen on the globe is the equatorial belt of calms between the trades, which has an annual movement northward and southward with the sun as already explained. To ascensional movements is to be ascribed part of the cloudiness of the southern and eastern sides of the winter cyclonic regions of the North Atlantic and North Pacific, and of the cyclonic regions of low summer pressure in the interior of Asia and other continents. On the other hand the comparatively small amount of cloud in the anticyclonic regions of the Atlantic and Pacific Oceans, and in the high-pressure regions of the interior of Asia and other continents during the cold months of the year, is due to the vast down-currents which occupy the centres of the anticyclones, and which become relatively drier as they descend owing to the increasing pressure to which the air is subjected.
Distribution of Atmospheric Pressure. - The importance of a knowledge of the distribution of atmospheric pressure, or of the mass of the atmosphere, over the globe in its varying amounts from month to month is self-evident. Observations teach us that winds are simply the movements of the atmosphere that set in from where there is a surplus towards where there is a deficiency of air ; and observations also teach that isobaric maps (i.e., maps showing the relative distribution of mean pressure) and maps showing the prevailing winds arc in accordance with each other. Since prevailing winds to a large extent determine the temperature and rainfall of the regions they traverse, isobaric maps may be considered as furnishing the key to the more important questions of meteoro logical inquiry. At the time of the first publication of isobaric snaps of the globe in 1868, it was impossible to do more than present the subject in its broad general features, owing to the scantiness and quality of the materials then existing. But since then meteorological stations have been largely multiplied in all parts of the civilized world, and the general adoption of the issue of storm warnings has necessitated the use of more accurate barometers and uniform methods of observing. Since there is thus now the means of a more exact representation of this fundamental datum of meteorology, we have prepared a new set of isobaric maps, showing the distribution of the earth's atmosphere and the prevailing winds for January (fig. 14), July (fig. 17), and the year. They have been constructed from mean values calculated for the same eleven years (1870-80 inclusive) as the isothermal maps figs. 10 to 13, pressure of 30'0 inches and upwards being represented by solid lines, and of 29.0 inches and under by dotted lines, while the arrows show the directions of the prevailing winds at the localities indicated by the respective arrow-points.
Mean Atmospheric Pressure in January (fig. 14). - In this month, when the influence of the sun on the northern hemisphere falls to the minimum, the greatest pressures are massed over the continents of that hemisphere, and the least pressures over the northern parts of the Atlantic and Pacific Oceans, over the Antarctic Ocean and southern hemisphere generally. In the southern hemisphere there are three patches where pressure rises to 30 inches, viz., in the Atlantic between South America and Africa, south of the Indian Ocean, and in the Pacific between Australia and South America.
In the northern hemisphere, on the other hand, pressure rises in Central Asia to upwards of 30.5 inches, the mean pressure for January being at least 30.4 inches at Peking, Semipalatinsk, and Yenisei, and fully 30.5 inches at Irkutsk and Nertchinsk, in the upper basin of the Amur. This is the region where the normal atmospheric pressure attains to a maximum which is much higher than is reached in any other region or at any other time of the year. It will be ohserved that this region of highest pressure occupies a position near the centre of the largest continent. The area of high barometer is continued westward through Europe, through the horse latitudes of the Atlantic to Carolina, and thence through the United States to California, whence it crosses the Pacific to Asia. This belt of high pressure thus completely encircles the globe, broadening as it passes the land and contracting as it crosses the ocean. Its greatest breadth is over Asia and its least over the Pacific, or where land and ocean attain respectively their maximum dimensions.
Pressures greatly under the average cover the northern portions of the Pacific and Atlantic and also the greaterpart of the Arctic regions. In the north of the Pacific the normal pressure falls to about 29'6 inches between Kamchatka and Alaska. In the north of the Atlantic, however, a still lower mean pressure obtains over a narrow belt stretching from Iceland to the south of Greenland, the normal at Stykkisholm in the north-west of Iceland being 29'385 inches, and at Ivigtut in Greenland 29'361 inches. This low average for Ivigtut is the lowest normal known to occur anywhere and at any season in the northern hemisphere, and it is significant that the place is immediately to the north of that part of the Atlantic where a considerable number of the storms which sweep over Europe have their origin, and where not a few of the storms which cross the Atlantic from America develop intensity.
It has been seen that the highest mean pressure occurs near the centre of the largest extent of land ; but as regards the two oceans the lowest pressure is met with in the northern division of the Atlantic, which is the lesser ocean. An inspection of fig. 14 shows, however, that the low-pressure area of the Atlantic is bounded to southward by systems of much higher pressures than are to be found in the Pacific. The result of this arrangement is that much stronger winds blow northward over the Atlantic and round upon Iceland ; and, as these more quickly advance into colder latitudes, there is thus a greater and more frequent concentration of vapour and lowering of the barometer in the north of the Atlantic. The heavy rainfall of north-western Europe may be referred to as confirming this view.
A belt of low pressure passes through the equatorial regions quite round the globe. This marks the well-known region of calms towards which on either hand the trade winds blow. In the Atlantic it lies quite north of the equator even in January, when the sun's course is farthest to southward, and it lies nearly parallel with the equator. On the other hand, in the Indian Ocean the position of the line of lowest pressure is to the south of the equator and.not parallel with it, but taking a slanting course from near the north of Madagascar towards Sumatra, thence towards the low pressure which prevails at this season in Australia ; its course is then a little to northwards, and crosses the Pacific to the central regions of South America. Its path is thus a devious one, being north of the equator only in the eastern part of the Pacific and in the Atlantic, but elsewhere to the south of it, being drawn farthest southward when under the influence of the regions of low pressure which now occupy central Australia, central and southern Africa, and central South America. In this trough of barometric depression nearly all the tropical storms of the Indian Ocean have their origin.
There are several important modifications of the isobaric lines as originally published. In 1868 the region of lowest pressure in the northern hemisphere in winter was represented as extending from Iceland to north-eastward ; now the area of lowest pressure is seen to extend from Iceland south-westward to Greenland. In connexion with this point Captain Doffmeyer discussed the weather of the North Atlantic during several winter months, and published the results in 1878, which conclusively- showed that the meteorology of Greenland and Iceland exerts on the distribution of atmospheric pressure a powerful influence not before properly recognized, resulting in the mean minimum of pressure being localized distinctly to the south-west of Iceland, and that in addition to this minimum there are two subordinate minima, one in Davis Straits and the other in the Arctic Ocean midway between Jan Mayen and the Lofoten Isles. The investigation further established the fact that, when any particular one of these three minima plays an important part, the other two either do not appear at all or occupy quite a subordinate place, and that according as one or other of these minima of pressure predominates so is the character of the weather, as regards mildness or severity, of the winter of northwestern Europe and regions surrounding the North Atlantic. As regards the British Islands, the displacement of the minima to westward of the position shown in fig. 14 means milder winter weather, whereas a position more in the direction of the north of Norway means severer winter weather.
Another change implying important consequences is seen in the United States, where, instead of one, two distinct centres of maximum pressure occur, or rather the high pressure of the western and central States is separated from that of the southeastern States by a region of lower pressure occupying the region of the Mississippi States. Professor Loomis first drew attention to this peculiarity in 1879 in an inquiry into the distribution of pressure over the United States, and established the fact that there are two distinct areas of high pressure, the larger having its centre in Utah, and the less ovcrspreading the greater portion of the southeastern and southern States, and that these two areas of high pressure are clearly separated from each other by a broad extensive region of lower pressure stretching in a south-western direction from the region of the great lakes to western Texas. The reason assigned by Professor Loomis is undoubtedly correct, that the relatively low normal pressure of the Mississippi States is due to the fact that the path usually taken by the barometric minima of American storms in the earlier part of their course is from Texas to the lakes. Since, on the other hand, the centres of comparatively few storms, with their low barometer readings, cross the southern and south-eastern States, the normal whiter pressure is higher there than it is along the Mississippi.
Another important modification occurs in India, where the isobar of 30 inches is deflected to the south-east toward Madras and thence towards the north-east to near Akyab in Arakan. This remarkable deflexion well shows the important influence exerted on the course of the isobar by large well-defined sheets of water and extensive -Lasts of land. The distribution of pressure here indicated, by which south of lat. 22° the normal pressure is considerab.ly higher in the east than in the west of India, has, through the agency of the winds resulting from it, the most intimate and vital bearings on the distribution of the winter rains and temperature over considerable portions of India ; and the same relations hold, but in a degree still more striking, in the meteorology of Ceylon.
The remarkable effect in interrupting or changing the course of the isobars is particularly well illustrated by the lines in the region of the Aral, Caspian, and Black Seas. As the point is of no small importance in meteorology, and is best illustrated by the Nediter ranean and the countries to the north of it, we subjoin a map of this part of Europe (fig. 15), showing the normal pressure with greater distinctness and fulness than could be shown on fig. 14, the isobars being drawn for every half-tenth of an inch.
Here we see two distinct areas of high pressure, the one in Hungary and the other in the Peninsula, where the normal pressure exceeds 30'20 incises. The latter is the larger of the two, and may lee regarded as the prolongation of the region of high pressure which characterizes the Atlantic immediately to the south-west at this season. The high-pressure area included within the isobar 30'15 inches is of peculiar interest. In the Peninsula it covers a pretty broad area, but to the north-east it contracts to a narrow m between the Bay of Biscay and the Gulf of Lyons, and again expands to north-eastward covering the distance from Carlsruhe to Modena, its prolongation eastward being there somewhat suddenly interrupted. At sonic distance to the eastward the second region of high pressure is met with, which is properly a part of the high pressure that overspreads the interior of the Old Continent in the winter months, its western limit being the isobar of 30.15 inches, which passes round by Pinsk, Cracow, Vienna, Laibach and the upper southern slopes of the basin of the Danube, Sebastopol, and thence southward in the direction of Cyprus.
The position of the latter of these regions of high pressure is approximately midway between the south coasts of Asia Minor and the Baltic. In other words, its position occupies the interior of this part of the Old Continent ; and it is instruetite to note that the position of the Black Sea and the Greek archipelago in the smith portion of this region pushes the isobar of 30-15 incises a good deal to northward. The position of the region of high pressure in the Peninsula, France, and Switzerland is also decidedly inland. It does not, however, exactly occupy the middle space of the land lying between the Mediterranean and the North Sea, owing no doubt to the circumstance that Use very steep barometric gradient from France to Iceland greatly lowers the pressure over the whole of the northern half of Prance. It follows that the abnormally high pressure which so remarkably characterizes the interior of the Ohl Continent during the cold months of winter is represented, though in a greatly reduced form, westwards through the central districts of that continent.
These two regions of high pressure are separated from each other by a large area of comparatively low pressure overspreading the greater portion of the Mediterranean Sea, - marked off in fig. 15 by the isobar of 30.10 inches, within which pressure is everywhere less than 30.10 inches. This region includes an area of still lower pressure within the isobar of 30'05 inches, bounded by Sicily, Corfu, Athens, and Crete. Hence the singularly low pressure which characterizes the northern part of the Atlantic at this season has its analogue in the south of Europe, which is unquestionably due to the higher temperature and larger humidity of the climates of southern Europe which they owe to the Mediterranean.
It is deserving of special notice that, while the increase of the normal pressure of January from Genoa to Geneva is 0051 inch, it is only 0.021 inch from Trieste to Rica, and that to the north of the Adriatic as far as latitude 50° pressure is considerably lower than obtains to the west and east of that region. An examination of the daily weather maps of Europe shows that not nnfrequently the storms of north-western Europe ou advancing as far to eastward as Denmark seem to connect themselves in some degree with Mediterranean storms prevailing at the time through a north and south prolongation of a system of low pressures. The comparative frequency with which this occurs is probably occasioned by the general drift to eastward of the atmosphere of Europe, considered as a whole, taken in connexion with the high mountainous ridge which bounds the Adriatic on its eastern side, from which it follows that the air overspreading the deep basin of the Adriatic is often highly saturated with vapour, and this highly saturated air is drawn northwards through central Europe when north-western storms of Europe with low barometric depression centres pass across Denmark and the Baltic. Thus the low normal pressure to the north of the Adriatic, separating the two regions of high pressure to the east and west of it, is in some respects analogous to the low normal pressure of the Mississippi valley, which separates the higher normal pressures of the Rocky Mountains and of the southeastern of the United States.
The influence of land and water respectively in the cold season of the year is well shown in fig. 16, which represents for every 0.020 inch the normal pressure over the British Islands in January, drawn from means calculated for two hundred and ninety-five stations.' It is in the winter months that the isobars of the British Islands crowd most closely together, and in accordance therewith strong winds are then most prevalent. The crowding of the isobars reaches the maximum in January, forming what is probably the steepest mean monthly barometric gradient that occurs at any season anywhere on the globe. The point, however, to which attention is here drawn is the remarkable influence of St George's Channel and the Irish Sea in diminishing the pressures as they cross these seas, and of the land in increasing the pressure, which is seen in the curves occupying approximately the central districts ing to the minimum in the southern hemisphere. With the solar conditions reversed, a comparison of figs. 14 and 17 shows that the distribution of atmospheric pressure in July is, considered in a broad sense, the reverse of what takes place in January.
In the southern hemisphere atmospheric pressure dining the winter season is above the general average of 30 inches between lat. 10° and 40° S. This belt of high pressure encircles the globe, and embraces four regions where pressure rises considerably above this general high average. These regions are in South Africa, about lat. 20°, where it rises to a little above 30.20 inches; in Australia, where it rises on the Murray river very nearly to 30.20 inches ; in South America, where in the basin of the La Plata, about lat. 30°, it rises to 30-13 inches ; and in the ocean to westwards, where it reaches 3002 inches. The point to be noted with respect to the position of these centres of high pressure at this season is that they occur over surfaces between latitudes 20° and 36°. As compared with January, pressure in July over nearly the whole of this broad belt of the southern hemisphere is about two-tenths of an inch higher, which is the simple result of season. A comparison of January and July shows that this large accession to the pressure of the southern hemisphere is accompanied by an extraordinary diminution of pressure over the continents of the northern hemisphere.
Now, just as the greatest excess of pressure during the winter of the northern hemisphere occurs in the continent of Asia, so the greatest diminution of pressure in the summer months takes place in the same continent. The position, however, of these two extremes is far from being in the same region or even near each other. In the Old Continent the maximum occurs in the valley of the upper Amur, where, at Nertchinsk, the normal pressure in January is about 30-500 inches ; whereas the lowest normal pressure in July is 29.412 inches, and occurs, so far as observation enables us to locate it, at Jacobabad on the west side of the basin of the Indus. The difference of these two normals is 1-188 inch ; and over no inconsiderable portion of central Asia the normal pressure of July is an inch less than that of January. In other words, the influence of the sun in summer as exerted on the temperature and aqueous vapour of the atmosphere and atmospheric movements resulting therefrom is so powerful as to remove a thirtieth part of the whole mass of the air from this extensive region.
The large extension in recent years of good meteorological stations over the Russian and Indian empires enables us to lay down with much greater precision than formerly the lines of pressure. Of the changes indicated by the new isobars, the most important perhaps is the position of the region of minimum pressure in Asia, which is now seen to occupy the basin of the Indus, and thence stretches over a somewhat broad region to westward nearly as far as the head of the Persian Gulf. The point is of no small importance in atmospheric physics, inasmuch as it places the region of least normal pressure in July as close geographically to the region where at the time terrestrial temperature is highest as the region of highest normal pressure in January is situated with respect to the region where in that month terrestrial temperature is lowest in Asia.
The July isobars of India are of singular interest, and imply consequences of the utmost practical advantage to the empire. From Cutch southward the normal pressure is everywhere higher, and considerably so, along the whole of the west than it is in the cast in the same latitudes, the difference being approximately half a tenth of an inch. This is represented on the map by the slanting of the isobars from north-west to south-east as they cross this part of India ; and it is to be noted that the cast and west coasts of Ceylon show the same manner of distribution of the pressure. The consequenee of this peculiarity in the distribution of the pressure is that the slimmer monsoon blows more directly from the ocean over western and southern India than would have been the case if the isobars had lain (Inc east and west, and thus probably precipitates in its course a more abundant rainfall over this part of the empire. But a more important consequence follows from the geographical distribution of the pressure over the valley of the Ganges. If the normal pressure there had diminished in the manner it does over India to the south of the Gangetic valley, the winds would have been south-westerly and the summer climate practically rainless. This, however, is not the case, but the normal pressure diminishes westwards along the valley of the Ganges, as the following mean July pressures will show : - Calcutta, 29.576 inches ; Patna, 29'535 inches ; Lucknow, 29.522 inches ; Roorkee, 29-505 inches ; and in crossing, westward into the Punjab pressure falls still lower - to 29.439 niches at Mooltan and 29.412 inches at Jacobabad. Indeed pressure in July is 0.220 inches lower at Jacobabad than at Sibsagar on the Brahmaputra, nearly in the same latitude. It necessarily follows from this distribution of the pressure that the summer monsoon, which blows northward over the Bay of Bengal, is deflected into an E.S. E. wind which fills the whole valley of the Ganges, distributing on its way a most generous rainfall over that magnificent region.
The influence of the land in lowering, the pressure in summer is well illustrated by the course of the isobars over western Siberia and Russia, where pressure is seen to fall relatively lowest along the middle line of the Old Continent. In this connexion it is interesting to note the course of the isobar of 29.90 inches over that part of Europe where the breadth of the land is considerably increased - between the Baltic and Constantinople. In contradistinction to this the influence of the Aral, Caspian, and black Seas in maintaining a higher pressure appears in the remarkable prolongation eastward of the isobars of higher pressure over the legion of these seas, being in striking contrast to the lower pressures which prevail to the north and south.
The lowering of the normal pressure is very decided in the inland regions of Spain, North Italy, and Scandinavia. The effect is most strongly seen in Spain, the largest and compactest of these legions. Thus, while the normal pressure diminishes between Lisbon and Barcelona from 30'0S6 to 30.048 incises, the sea-level pressure at Madrid falls nearly to 30'000, and the pressure at Saragossa and -Valladolid is nearly as low. This lowering of the pressure over the interior influences materially its summer climate. As remarkable an illustration of the principle as can be pointed to anywhere is seen in the north of Italy ; for, while the normal pressure at Moncalieri is 29.941 inches, at Genoa on the coast the relatively high normal of 29.992 inches is maintained, the distance of the two places being about 40 miles. To the cast pressure rises to 29-970 inches at Venice, and to westward to 30.023 inches at Geneva. Over Scandinavia, along the west coast from time Arctic circle southward, the normal pressure equals or exceeds 29-S0 inches, the variation being comparatively small ; and along the coast from the head of the Gulf of Bothnia to the south-east of Sweden pressure also exceeds 29'80 inches, and the increase from north to south proceeds at a slow rate. In, however, the strictly inland districts to the north-east of Christiania, which lie immediately to the cast of the Scandinavian mountains, and sheltered by that lofty range from the winds of the Atlantic, pressure is considerably lower than it is along the east and west coasts of the peninsula. Owing to this peculiar distribution of the pressure, the winds which necessarily result from it give a much finer summer climate to the south-east of -Norway and to the strictly inland part of Sweden than would otherwise be the case.
The remarkable carving northward of the isobar of 2980 incises so as to include Lapland within it points probably to the influence of the White Sea and the wonderful lake system of Lapland in maintaining a higher summer pressure over that country, by which the northerly winds that blow towards the low-pressure region of Central Asia, to the serious deterioration of the summer climate of northern Siberia, do not extend so far to westward as Lapland.
The distribution of the normal pressure over North America is quite analogous to what prevails over Asia, but, the continent being less, the diminution of pressure in the interior is also correspondingly less. The highest normal pressure, 30-077 inches, is fonnd in the south-east in Florida, and the lowest, 29-780 inches, in Utah, the difference being thus 0-297 inch. Another region of relatively high pressure is in the north-western States and British Columbia to the north ; the maximum, near the mouth of the Columbia river, reaches 30'062 inches, being thus nearly as high as what occurs in Florida. These two regions are merely extensions of important high-pressure areas which at this seasons are highly characteristic features of the meteorology of the North Pacific and North Atlantic respectively.
Of these two regions of high pressure the one overspreading the Atlantic between the United States and Africa is the more striking, being not only the region where pressure is highest anywhere on the globe during the months of June, July, and August, but where the normal pressure reaches the highest point attained at any season over the ocean. The highest point reached by the normal pressure over the land at any season occurs, as has been pointed out, near the centre of Asia, or approximately in the middle region of the largest continuous land surface on the globe during the coldest months of the year. On the other hand, the highest pressure over the ocean occurs during the warmest months of the year, and not over the largest water surface, bat in the middle regions of the North Atlantic, where the breadth is only about half that of the water surface of the North Pacific.
From the essential differences between these two sets of phenomena it may be inferred that the extraordinarily high pressure which is so marked a feature of the meteorology of Central Asia during the cold months of the year is a direct consequence of the lowering of the temperature of the land of Asia and of the atmosphere resting on it during the time of the year when the effects of solar radiation are at the annual minimum, and of terrestrial radiation at the annual maximum. But the determination of the place and time of highest pressure over the ocean must be regarded as indirectly brought about. The physical conditions under which it occurs are these : - it happens (1) at the time of the year when the earth presents the largest surface of laud to the sun, and (2) over that part of the ocean which is most completely surrounded by these highly heated land surfaces. This high summer pressure of the Atlantic has its origin in the upper currents of the atmosphere.
Mean Atmospheric Pressure for the Year. - The distribution of the annual atmospheric pressure may be considered as representing the sums of the influences directly and indirectly at work throughout the year in increasing or diminishing the pressure of the atmosphere. There are two regions of high pressure, the one north and the other south of the equator, which pass completely round the globe as broad belts of high pressure. The belt of high pressure in the southern hemisphere lies nearly parallel to the equator, and is of nearly uniform breadth throughout ; but the belt north of the equator has a very irregular outline, and shows great differences in its breadth and its inclination to the equator. These irregularities wholly depend on the peculiar distribution of land and water which obtains in the northern hemisphere.
These two zones of high pressure enclose between them the comparatively low pressure of the tropics, through the centre of which runs a narrower belt of still lower pressure, towards which the tradewinds on either hand blow. Considered in a broad sense, there are only three regions of low pressure, the equatorial one just referred to, and one round each pole bounded by or contained within the zones of high pressure just described. The most remarkable of these, so far as it is known, is the region of low pressure about the south pole, which remains low throughout the year, playing the principal role in the wind systems of the Antarctic zone, in its heavy snowfall and rainfall, and in the enormous icebergs which form so striking a feature of the water of the Southern Ocean.
The depression around the north pole contains within its area two distinct centres of still lower pressure, the One filling the northern part of the Atlantic and the other that of the Pacific. Of these two the low-pressure area round Iceland is the deeper' and is probably occasioned by the steeper barometric gradients and stronger winds which prevail over the North Atlantic. The broad equatorial zone of low pressure also contains two distinct regions characterized by still lower' pressures. The larger of the two stretches across southern Asia from Assam to the head of the Persian Gulf, and is entirely duo to the very low pressures which form so marked a feature in the summer meteorology of that part of Asia. The regions of the middle Indus and upper Ganges occupy the centre of this low-pressure area, where normal pressure falls short of 29'80 inches. The second area of lowest equatorial pressure is in the centre of Africa.
It may be here pointed out that the whole of these areas of low mean annual pressure possess the common characteristic of an excessive amount of moisture in the atmosphere. The Arctic and Antarctic zones of low pressure, and the equatorial low-pressure zone generally, may be regarded as all but wholly occasioned by the Con• rtratively large amount of vapour in their atmosphere. As regards the region of low pressure of southern Asia in summer, it is remarkable that, while the eastern half which overspreads the valley of the Ganges is characterized by a moist atmosphere and large rainfall, the western half of it is singularly dry and practically rainless, and that the central portion of this remarkable depression occupies a region where at the time the climate is one of the driest and hottest anywhere to be found on the globe. Hence, while the vapour is the more important of the disturbing influences at work in the atmosphere, the temperature also plays no inconspicuous part directly in destroying atmospheric equilibrium, front which result winds, storms, and many other atmospheric changes.
The Prevailing Winds of the Globe. - If atmospheric pressure were equal in all parts of the earth we should have the physical conditions of a stagnant atmosphere. Such, however, is not the case. Let there be produced a concentration of aqueous vapour over a particular region, or let one region show a higher temperature than what prevails around it, then from the different densities, and consequently different pressures thereby produced, the equilibrium of the atmosphere is destroyed, and, as might be expected from the laws of aerial fluids, movegiven, as well as every isobaric map which has been made of the atmospheric movements which result from the disturbance of the equilibrium of the atmosphere indicated by the isobaric maps for that season and region.
All winds maybe regarded as caused directly by differences of atmospheric pressure, just as the flow of rivers is caused by differences of level, the motion of the air and the motion of the water being both referable to gravitation. The wind blows from a region of higher towards a region of lower pressure, - in other words from where there is a surplus to where there is a deficiency of air ; and this takes place whether the differences of pressure be measurable by the barometer, as is generally the case, or not readily measurable, as in the case of sea breezes, squalls, and sudden gusts of wind which are of short duration.
So far as is known, differences of atmospheric pressure, and consequently all winds, originate in changes occurring either in the temperature or the humidity of the air over restricted regions. Thus, if two regions contiguous to each other come to be of unequal temperature, the air of the warmer region, being specifically lighter, will ascend, and the heavier air of the colder region will flow in below to take its place. Of this class of winds the sea and land breezes are the best examples. Again, if time air of one region comes to be more highly charged with aqueous vapour than the air of surrounding regions, the air of the more humid region being lighter will ascend, while the heavier air of the drier regions will flow in below and take its place. Since part of the vapour will be condensed into cloud or rain as it ascends, heat is thereby disengaged, and the equilibrium still further disturbed. In this way originate gales, storms, tempest; hurricanes, and all the more violent commotions of the atmosphere, except seine of the forms of the whirlwind, such as dust storms, in the production of which very great differences of temperature are more immediately and exclusively concerned.
The Trude- Winds. - From fig. 14, giving the isobarics for January, it is seen that atmospheric pressure in the Atlantic is lower near time equator than it is to north and south of it; and the arrows indicate that to the north of the tract of lowest pressure N.E. winds prevail and to the south of it S.E. winds. These are the well-known N.E. and S.E. trade-winds, which thus blow from regions of high pressure towards the tract of lower pressure situated midway between them. The trade-winds do not blow directly to where the lowest pressure is, but in a slanting direction at an angle of about half a degree. The deviation from the direct course is due to the influence of the rotation of the earth on its axis from west to east, - an influence to which all winds and all currents of the ocean are subject.
In virtue of this rotation, objects on the earth's surface at the equator are carried round towards the east at the rate of about 17 miles a minute. On receding from the equator, however, this rate of velocity is being continually diminished, so that at 60° N. lat. it is only about Si miles a minute, and at the poles nothing. From this it follows that a wind blowing along the earth's surface in the direction of the equator is constantly arriving at places which have a greater eastward velocity than itself. As the wind thus lags behind, these places come up, as it were, against it, the result being an east wind. Since, therefore, the wind north of the equator is under the influence of two forces - one, the low pressure near the equator, drawing it southwards, and the other, time rotation of the earth, deflecting it eastwards - it will, by the law of the composition of forces, take an intermediate direction, and blow from north-east. For the same reason, south of the equator the south is deflected into a south-east wind.
In the Atlantic the north trades prevail between latitudes 7° and 30° N., and the south trades between latitudes 3° N. and 25° S. These limits are not stationary, but follow the sun, being farthest to the south in February and to the north in August. The tract of low pressure between these wind systems is named the region of calms, owing to the calm weather which often prevails there, and it 18 also characterized by the frequent occurrence of heavy rains. This region of calms varies its position with that of the sun, reaching its most northern limit, lat. 11° N., in August, and its most southern, lat. N., in February.
Its breadth varies from 3° to 8°, and it lies generally parallel to the equator. It is to be noted that, in the Atlantic, the region of calms is at all seasons north of the equator.
North and south trades also prevail in the Pacific Ocean, separated by a region of calms, which would appear, how-ever, to be of less breadth and to be less clearly defined than is the region of calms in the Atlantic. In the eastern portion of the Pacific the region of calms lies at all seasons to the north of the equator, but in the western division it is considerably south of the equator during the summer months of the southern hemisphere, this southerly position being in all likelihood occasioned by the extraordinarily high pressure in Asia in its relations to the low pressure in the interior of Australia at this season. During the summer months of the northern hemisphere the region of calms wholly disappears from the Indian Ocean and from the western part of the Pacific Ocean, there being then an unbroken diminishing pressure from the latitude of Mauritius and Central Australia northwards as far as the low pressure of Central Asia.
Regions of light and variable winds and calms occur at the higher limits of the north and south trades. Except in the Pacific, where, owing to the greater breadth of that ocean, they spread over a considerable extent, these regions appear but in circumscribed patches, such as characterize the meteorology of the North and South Atlantic about latitudes 26° to 36°. Of these regions of cairns the most important is that marked off by the high pressure in the North Atlantic, between the United States and Africa. This is the region of the Sargasso Sea, where the weather is characterized by calms and variable winds, and the ocean by its comparatively still waters. These are known to seamen as the " horse latitudes," and are essentially different from the equatorial region of calms. The latter, as has been stated, is the region of low pressure at the meeting of the north and south trades, where the climate is distinguished for its general sunlessness and heavy rainfall. On the other hand, the calm regions in the Atlantic and Pacific Oceans about the tropics have an atmospheric pressure abnormally high, clear skies, and the weather generally sunny and bright, with occasional squalls.
Numerous observations made in all parts of the globe establish the fact that, while the surface winds within the tropics are directed towards the equatorial region of calms in such a manner that the general intertropical movements of the atmosphere or prevailing winds are easterly, the prevailing winds of the north and south temperate zones are westerly. The westing of these great aerial currents is due to the same cause that gives easting to the trade-winds, viz., the rotation of the earth round its axis. For, as an aerial current advances into higher latitudes, it is constantly arriving at regions having a less rotatory velocity than itself ; it thus outstrips them and leaves them behind ; in other words, it blows over these places as a westerly wind.
While, however, the general prevalence of westerly winds has been established over the extratropical regions of Europe, Asia, Africa, America, and Australia, the directions which in different seasons and at different places are actually found to prevail often differ very widely from west. An examination of the winds at one hundred and fifteen places pretty well distributed over the northern hemisphere reveals the instructive fact that almost every place shows two maximum directions from which winds blow more frequently than from the other directions, and that one of these two directions shows a considerable excess over the other. Thus, for example, the following are, on a twenty years' average, the number of days at Greenwich each wind prevails during the year : - N., 11; N.E., 49; E., 23; S.E., 21; S., 34; S.W., 103; W., 38; N.W., 24; and calms, 32. Hence S.W. and N.E. winds are there more prevalent than winds from any other direction, and of these two winds the greater maximum direction is S.W. If the two maximum directions be sorted into groups, then the greater maximum direction occurs as follows :- from S.S.W. to W. at 47 places „ W.N.W. „ N. „ 33 „ „ N.N.E. „ E. „ 19 „ „ E.S.E. „ S. „ 16 „ and the other maximum direction is from S.S.W. to W. at 20 places „ W.N.W. „ N. „ 22 „„ N.N.E. „ E. „ 33 „„ E.S.E. „ S. „ 32 „ This result of observation, so different from what was long accepted as being in accordance with the generally received theory of the movements of the atmosphere, teaches the important lesson that the region towards which the extra-tropical winds of the northern hemisphere are directed is not the region of the north pole.
Prevailing Winds in January. - On examining fig. 14, which shows the distribution of atmospheric pressure in January, it is seen that pressure is abnormally low over the northern portion of the Atlantic - the lowest occurring between Iceland and South Greenland - from which it rises as we proceed in a S.W. direction towards America, in a S. direction over the Atlantic, and in a S.E. and E. direction over Europe and Asia. Nov what influence has this remarkable atmospheric depression on the prevailing winds over this large and important part of the earth's surface ? The arrows in the figure, which indicate the prevailing winds, and which have been laid down from observations, answer this question.
At stations on the east side of North America the arrows show a decided predominance of north-west winds; at the more northern places the general direction is more northerly, whereas farther south it is more westerly. In the Atlantic between America and Great Britain, in the south of England, in France and Belgium, the direction is nearly S.W. In Ireland and Scotland it is W.S.W.; in Denmark and the north-west of Russia S.S.W. ; from St Petersburg to Tobolsk S.W. ; on the west of Norway generally S,S.E.; and in Greenland, the north of Iceland, and about Spitzbergen N. E. Hence all the prevailing winds in January over this extensive portion of the globe may be regarded as the simple expression of the difference of atmospheric pressure which prevails over the different parts of the region. In truth the whole appears to flow vorticosely, or in an in-moving spiral course, towards the region of low pressure lying to the south-west of Iceland, and extending eastward over the Arctic Sea north of Russia. The only marked changes in these directions of the wind thus broadly sketched out are the deflexions caused by the various mountain systems which lie, so to speak, embedded in these vast aerial currents ; of these the winds in the south of Norway afford excellent illustrations.
The influence which this peculiar distribution of the pressure over the north of the Atlantic exercises in absolutely determining the winter climates of the respective countries is most instructive. It is to this low pressure, which draws over the British Islands W.S.W. winds from the warm waters of the Atlantic, that the open, mild, and, it must be added, rainy winters of these islands are due. The same region of low pressure gives Russia and Western Siberia their severe winters ; and it is the same consideration that fully explains the enormous deflexion of the isothermal lines from Norway eastwards and southeastwards over the Old Continent. Finally, the same low pressure draws over British America and the United States, by the N.W. winds which it induces, the intensely dry cold air-current of the Arctic regions. At Portland, Maine, which is swept by these cold north-westerly winds, the normal temperature in January is 23'6, whereas at Corunna, on the coast of Spain, in nearly the same latitude, where south-westerly winds from the Atlantic prevail, the mean temperature of the month is 49°.l, or 25°'5 higher.
The region of low atmospheric pressure in the north of the Pacific is accompanied by prevailing winds over the region embraced by it and by climatic effects in all respects similar to the above. In Vancouver Island the prevailing winds in January are S.W., at Sitka E.S.E., on Great Bear Lake E.N.E., in Alaska N.E., in Kamchatka N.N.E., and in Japan N.W. In accordance with these winds the winter climate of Vancouver and adjoining regions is mild and humid, and that of the north-east of Asia dry and intensely cold.
On the other hand, abnormally high pressure rules over the continent of Asia at this season, and as regards this region of high pressure the arrows represent the winds as blowing outwards from it in alI directions. Over the interior of Asia, where the highest normal pressures are, observations show a marked prevalence of calms and light winds, but around this central region the prevailing winds in January are - at Calcutta N., at Hong-Kong E.N.E., at Peking N.W., on the Amur W.N.W., S.E. at Nijnikolynisk and S.S.W. at Ustjansk (in the north of Siberia), and at Bogoslovsk S.W. Hence from this extensive region, where pressure is abnormally high, or where at this season there is a large surplus of air, the prevailing winds flow outwards in all directions towards the lower pressure which surrounds it. Owing to the excessive dryness of the air of Central Asia, terrestrial radiation is less obstructed there than anywhere else on the globe, and consequently the temperature falls very low, the mean of January at Werchojansk being - 55°•8, which is the lowest mean monthly temperature known to occur on the earth's surface. And, since the winds blow outwards from the dry cold climates of the interior, temperatures are low, even on the coasts. Of this China affords good illustrations. Thus the mean January temperature of Peking is 22°•7 and of Zi-ka-Wei, near Shanghai, 35°'4, whereas at Corfu and Alexandria the normal temperatures for January are respectively 500-9 and 58'0, or 28°.2 and 22•6 higher than in corresponding latitudes on the coast of China.
The winds of the United States in winter, taken in connexion with the peculiar distribution of pressure already described, are very interesting. There are two regions of high pressure, one in the south-eastern States and the other and larger one in the States around Utah ; and between these there is interposed a trough of lower pressure extending from Chicago to the south-west of Texas. On the western side of this depression the winds are northwesterly, but to the east of it they become W., W.S.W., and in seine places S.W., and again on nearing the Atlantic seaboard they become north-westerly. In connexion with the region of higher pressure in the west, the prevailing winds are seen to flow outward from it. The normal pressure diminishes everywhere to southward of a line drawn from the Canaries to Bermuda, thence westward in nearly the same latitude to Texas, and then to west-northwest to San Francisco. The tract of lowest pressure stretches from the basin of the Amazon in the direction of the isthmus of Panama in about latitude 8° N., and thence is continued westward for a considerable distance into the Pacific in nearly the same latitude. It follows from this distribution of the pressure that the north trades in a more or less modified form prevail over South America to the north of the Amazon, and in the Pacific to the north of lat. 8° N., probably as far to westward as long. 150° W.
The low-pressure systems which prevail during the summer months in South America and South Africa have each its corresponding system of winds all round. It is, however, in Australia, as being the most compact and isolated continent, that the influence of the summer sun in lowering the pressure is best illustrated. In that continent the lowest pressure occurs in the region situated about midway between the north coast and the tropic of Capricorn, over which the normal pressure does not exceed 29-80 inches. Further, everywhere in Australia pressure diminishes from the coast on advancing upon the inland districts. It follows from this disposition of the pressure that all round the island the prevailing winds in summer blow from the sea towards the interior ; and accordingly it is in these months that the greater part of the rain falls. From the low pressure of the interior southwards to Bass's Straits pressure rises continuously, the increase in the normal over this space being about 0.200 inch. To northward it also rises continuously to beyond the north of China, the increase on this side being about of an inch. In this case the greater part of the increase occurs over the continent, the rate of increase from the north of Australia to the Philippine Islands being only about the rate of increase which obtains southward towards Bass's Straits. It will be shown when the subject of the rainfall is examined that it is the relative excess of these high pressures, the one in the south of Australia and the other in the south-east of Asia, that determines the position of the area of low pressure in Australia in particular years, and with that position the degree and extent to which the whole of the northern portion of Australia is watered by the rainfall. Thus, when pressure is more than usually high in the south-east of Asia, and either low or not excessive in the south of Australia, then the low-pressure region is pushed farther southward into the interior, and with it the rainfall spreads inland over a wider area and to a greater depth.
Prevailing Winds in July. - In the winter of the southern hemisphere, the geographical distribution of pressure is exactly the reverse in Australia of what obtains during the summer months. Everywhere all round it increases on advancing from the coast into inland districts. The lowest pressure, about 30.00 inches, occurs on the north coast, and the highest over the basin of the Murray river and its affluents, where it rises generally to 30.18 inches. On the south coast it is generally about 30-12 inches, failing, however, at Gabo Island, in the extreme south-east, to 30.050 inches, and to 29.836 in the south of New Zealand. From the Murray river the diminution of pressure is continuous to the north, even to the low pressure of Central Asia. From this arrangement of the pressure, the prevailing winds blow from the interior towards the surrounding ocean all round Australia, with the single exception of the extreme south-west of the continent, where the prevailing winds are south-westerly, being here essentially an outflow of the high pressure which overspreads the Indian Ocean to the westward. As these S.W. winds are from the ocean, the rainfall at Perth in July is fully 6 inches, and it is high over south-western districts of West Australia. The prevailing winds round Australia are S.E. on the north coast, S.W. at Brisbane, W.N.W. at Sydney, N. at Melbourne, N.E. at Adelaide. These all represent an outflow from the high-pressure regions of the interior modified by the influence of the earth's rotation, and, in correspondence with the reversal of the distribution of the pressure, are directions the reverse of the prevailing winds of January.
In July the central and southern parts of Asia are highly heated by the summer sun, and, besides, the rainfall over southern parts is excessive. Consequently atmospheric pressure is very low, being fully 0.40 inch lower in the Punjab than it is in the south of Ceylon. From the interior pressure rises continuously on advancing to the eastward, southward, westward, and northward, and from all these directions the prevailing winds of summer flow inwards upon the interior, and these bring rain or parching drought according to the vapour they bring from the ocean they have traversed, and according as they advance into warmer or colder regions. The prevailing summer winds of Asia, being an inflow inwards upon the interior, have, generally speaking, exactly the reverse direction of that prevailing in winter.
The winds of Europe are mainly determined by the extraordinarily high pressure of the Atlantic in its relations to the low-pressure systems of Central Asia and Central Africa at this time. The winds in the Spanish Peninsula are north-west ; in the north of Africa they are northerly, and again north-westerly in Syria. The winds of the British Islands and western Europe have less southing and more northing than the prevailing winds of winter, and to the east of long. 40° E. they become decidedly north-west. It is to the Atlantic origin of these winds that the summer climates of these large and important regions owe the comparatively large rainfall of this season, it being at this time that the rainfall reaches the annual maximum. The bearing of the low-pressure areas and mountain systems of the north of Italy and Scandinavia on the climates of these countries will be afterwards referred to.
The centre of lowest pressure in North America is over the central States about Utah, from which it rises all round, least to northward and most in south-easterly and north-westerly directions. In California N.W. winds necessarily blow inwards upon this central low-pressure area; and, as these winds pass successively over regions the temperature of which constantly increases, the summer climate is rainless. On the other hand, southerly and south-easterly winds from the Gulf of Mexico blow up the western side of the basin of the Mississippi inwards upon the low-pressure area of the centre, depositing in their course, in a rainfall more or less abundant, the moisture they have brought from the Gulf. To the north of lat 50°, and to westward of Hudson's Bay, the prevailing winds become easterly and north-easterly, distributing over Manitoba, Saskatchewan, and neighbouring regions, as they continue their westerly course towards the low-pressure area, the rainfall they have transported thither from the wide expanse of Hudson's Bay. An attentive examination of the arrows of fig. 17 shows that the prevailing winds over all the States to the east of the Mississippi river are rather to be regarded as an outflow from the region of very high pressure over the Atlantic to southeastward. Thus in Florida the winds are S.E., in the southern States S., and in the lake region, in the New England States, and on the Atlantic seaboard S.W. Since the origin of these winds is thus essentially oceanic, and since in their course northwards no mountain range crosses their path, the whole of this extensive region enjoys a large but by no means excessive rainfall, which, taken in connexion with the temperature, renders the summer climate of these States one of the best to be met with anywhere on the globe for the successful prosecution of agricultural industries.
The remarkable protrusion of high pressures from the southern hemisphere, where they are massed at this time of the year, northwards into the Atlantic is, as has already been referred to, one of the outstanding features of the meteorology of the summer months of the northern hemisphere. In the central area of this large region the climate is remarkable for its prevailing calms, light winds, occasional squalls, and clear skies. From this comparatively calm space the wind blows outwards in all directions towards and in upon the surrounding regions of low pressure. These winds, owing to the high temperature, clear skies, and strong sunshine of the region from which they issue, carry with them a great amount of vapour near the surface, by which to a large extent the north of South America, the east of North America, the greater part of Europe, and a large portion of Africa are watered. The prevailing winds over this region are further interesting, not merely from the striking illustration they give of the intimate relation of the winds to the distribution of the pressure, but as being of no small importance in determining the best routes to be taken over this great highway of commerce, and the more so inasmuch as the currents of the ocean are coincident with these prevailing winds.
In the Antarctic regions, or rather to the south of lat. 45° S., the normal atmospheric pressure is low at all seasons, there being a gradual diminution of pressure to 29•20 inches about lat. 60° S. Pressure is probably even still lower nearer to the south pole, as seems to be indicated by the observations made by Sir James C. Ross, and in the " Challenger " and other expeditions. Over this zone the prevailing winds are W.N.W. and N.W. This is the region of the "brave west winds," the "roaring forties" of sailors, which play such an important part in navigation, and which determine that the outward voyage to Australia be round the Cape of Good Hope and thence eastward, and the homeward voyage eastward round Cape Horn, the globe being thus circumnavigated by the double voyage. That the general drift of these winds is inwards upon the south pole is strongly attested by the existence of the enormously thick wall of ice which engirdles these regions, from which are constantly breaking away the innumerable icebergs that cover the Southern Ocean, none of which is ever seen of a calculated thickness less than 1400 feet. The snow and rainfall which must take place in the south polar regions for the formation of icebergs of such a thickness must be peculiarly heavy, but not heavier than might be expected from the strength and degree of saturation of the "roaring forties " which unceasingly precipitate their moisture over these regions.
To sum up : - so far as the prevailing winds are concerned, it has been shown that where pressure is high, that is to say, where there exists a surplus of air, out of such a region winds blow in all directions ; and, on the other hand, where pressure is low, or where there is a deficiency of air, towards such a region winds blow from all directions in an in-moving spiral course. This outflow of air-currents from a region of high pressure upon a region of low pressure is reducible to a single principle, viz., the principle of gravitation. Given as observed facts the differences of pressure, it is easy to state with a close approximation to accuracy what are the prevailing winds, before calculating the averages from the wind observations. Indeed so predominating is the influence of gravitation where differences of pressure, however produced, exist that it may practically be regarded as the sole force immediately concerned in causing the movements of the atmosphere. If there be any other force or forces that set the winds in motion independently of the force called into play by differences of mass or pressure, their influence must be altogether insignificant as compared with gravitation.
It has been abundantly proved that the wind does not blow directly from the region of high towards that of low pressure, but that, in the northern hemisphere, the region of lowest pressure is to the left of the direction towards which the wind blows, and in the southern hemisphere to the right of it. This direction of the prevailing wind with reference to the pressure is in strict accordance with Buys Ballot's Law of the Winds, which may be thus expressed :the wind neither blows round the centre of lowest pressure in circles, or as tangents to the concentric isobaric curves of storms or cyclones, nor does it blow directly towards the centre ; but it takes a direction intermediate, approaching, however, more nearly to the direction and course of the circular curves than of the radii to the centre. The angle formed by a line drawn to the centre of lowest pressure from the observer's position and a line drawn in the direction of the wind is not a right angle, but an angle of from 60° to SO°.
From its importance in practical meteorology Buys Ballot's law may be stated in these two convenient forms. (1) Stand with your back to the wind, and the centre of the depression or the place where the barometer is lowest will be to your left in the northern hemisphere, and to your right in the southern hemisphere. This is the rule for sailors by which they are guided to steer with reference to storms. (2) Stand with the high barometer to your right and the low barometer to your left, and the wind will blow on your back, these positions in the southern hemisphere being reversed. It is in this form that the prevailing winds of any part of the globe may be worked out from the isobaric charts (figs. 14 and 17).
From the all-important consequences which flow from the geographical distribution of the pressure it is evident that the regions of low and of high normal pressure must be regarded as the true poles of the prevailing winds on the earth's surface, towards which and from which the great movements of the atmosphere proceed. From the unequal distribution of land and water, and their different relations to solar and terrestrial radiation, it follows that the poles of pressure and of atmospheric movements are, just as happens with respect to the poles of temperature, very far from being coincident with the north pole. Thus during the winter months the regions to which the origin of the great prevailing winds of the northern hemisphere are to be referred are Central Asia, the region of the Rocky Mountains, and the horse latitudes of the Atlantic, and the regions towards and in upon which they flow are the low-pressure systems in the north of the Atlantic and Pacific Oceans, and the tract of low pressure within the tropics towards which the trade-winds blow. In the summer months the reversed conditions of pressure-distribution then observed are attended with corresponding changes in the prevailing winds ; and, generally speaking, if the south polar region be excepted, the poles of highest and lowest pressure and atmospheric movements are at no time coincident with the north pole. It is this consideration which affords the true explanation why prevailing winds at so large a proportion of stations in the northern hemisphere do not blow in the directions in which true equatorial and polar winds should blow.
The causes which bring about an unequal distribution of the mass of the earth's atmosphere are mainly these two - the temperature and the moisture of the atmosphere considered with respect to the geographical distribution of land and water. Owing to the very different relations of land and water to temperature, as already stated, the summer temperature of continents greatly exceeds that of the ocean in the same latitudes. Hence the abnormally high temperature which prevails in the interior of Asia, Africa, America, and Australia during their respective summers, in consequence of which the air, becoming specifically lighter, ascends in enormous columns thousands of miles in diameter. On arriving at the higher regions of the atmosphere it flows over neighbouring regions where the surface temperature is lower, and thus the atmospheric pressure of the highly heated regions is diminished.
Surface winds set in all round to take the place of the air removed from the continents by these ascending currents, and since these necessarily are chiefly winds from the ocean they are highly charged with aqueous vapour, by the presence of which, and by the condensation of the vapour into cloud and rain, the pressure over continents at this season is still further and very largely diminished. Air charged with vapour is specifically lighter than when without the vapour ; in other words, the more vapour any given quantity of atmospheric air has in it the less is its specific gravity ; and, further, the condensation of vapour in ascending air is the chief cause of the cooling effect being so much less than that which would be experienced by dry air. From these two principles, which were established by Dalton, Joule, and Sir William Thomson, it follows that the pressure of vapour in the air, and its condensation, exercise a powerful influence in diminishing the pressure. The great disturbing influences at work in the atmosphere are the forces called into play by its aqueous vapour ; and it is to these, co-operating with the forces called into play by the differences of temperature directly, that the low normal pressure of the continents during the summer is to be ascribed. The degree to which the lowering of the pressure takes place is, as was to have been expected, greatest in Asia, the largest continent, and least in Australia, the smallest continent, while in America it is intermediate.
The influence of the aqueous vapour in diminishing the pressure is well seen in the belt of calms in the tropics between the north and the south trade-winds. Since these winds import into the belt of calms the vapour they have taken up from the sea on their way thither, the climate is characterized by a highly saturated atmosphere and heavy rains. Again the air in regions near the Atlantic contains much more vapour and is of a higher temperature during winter than is observed at places in the interior of continents in the same latitudes. It follows thus that the air over the north of the Atlantic and the regions adjoining is specifically lighter than in the regions which surround them. We have here therefore the physical conditions of an ascending current ; and it is plain that the strength of this current will not merely be kept up but increased by the condensations of the vapour into cloud and rain which take place within it, by which a higher temperature and a greater specific lightness are maintained at the surface of the earth and at various heights in the atmosphere than exist over surrounding regions at the same heights. Accordingly it is seen from the winter isobars that an enormous diminution of pressure occurs over these regions, and also over the north of the Pacific and the Antarctic, as compared with the continents.
Since, on the other hand, dry and cold air is specifically heavy, the winter isobars show that where temperature is low and the air very dry pressure is high. Of this Asia and North America are striking examples during December, January, and February, and Australia, South Africa, and South America during June, July, and August.
Since vast volumes of air are thus poured into the region where pressure is low without increasing that pressure, and vast volumes flow out of the region where pressure is high without diminishing that pressure, it necessarily follows that the volumes of air poured into the region of low normal pressure do not accumulate over that region, but must somehow escape away into other regions, and that the volumes of air which flow out from the region of high normal pressure must have their place supplied by fresh accessions of air poured in from above. That the same law of relation observed between sea-level pressures and surface winds obtains between pressures at different heights and winds at the same heights is simply a necessary inference. We are therefore justified in expecting that ascending currents will continue their ascent till a height is attained at which the pressure of the air composing the currents equals or just falls short of the pressure over the surrounding regions at that high level. On reaching this height the air, no longer buoyed up by a greater specific levity than that of the surrounding air, will cease to ascend, and expanding horizontally will thenceforth flow over as an upper current towards those regions which offer the least resistance to its course ; that is to say, it will flow over upon those regions where, at that height, pressure happens at the time to be least. Now from the known densities of air of different temperatures and humidities it is evident that the overflow of the upper current will take place towards and over that region or regions the air of which in the lower strata of the atmosphere happens to be colder and drier than that of the other surrounding regions, - because, being denser, a greater mass of air is condensed or gathered together in the lower strata of the atmosphere, thus leaving a less mass of air, or a diminished pressure, in the higher region of the upper current.
If this be so, then the extraordinarily high pressure of Central Asia during winter is to be ascribed to these two causes : - (1) the low temperature and excessive dryness of the air of this extensive region ; and (2) its relative proximity to the low pressure of the Atlantic to the northwest, the low pressure of the Pacific to the north-east, and the low pressure of the belt of calms to the south. Similarly, since in summer the temperature of air resting over the Atlantic between Africa and the United States is much lower than that of the land, the ascending currents which arise from the heated lands of Africa, Europe, and North and South America, as well as from the region of calms immediately to the south, all of which are remarkable for a low normal pressure, will on reaching the upper regions of the atmosphere flow towards this part of the Atlantic, because there, the temperature being lower and the density of the air composing the lower strata being greater, pressure in the upper regions is less. And, since the surface winds are constantly flowing outwards from this region of abnormally high pressure, thus draining away the air poured down upon it by the upper currents which converge upon it, extreme saturation does not take place, and the air consequently is relatively dry and cool. That this view generally represents the movements of the upper currents has been strongly confirmed within the last few years by Professor Hildebrandsson and Clement Ley in their researches into the upper currents of the atmosphere based on observations of the cirrus cloud.
From these considerations it may be concluded that the winds which prevail near the earth's surface are known from the isobaric lines, the direction of the wind being from regions where pressure is high towards regions where it is low, in accordance with Buys Ballot's law ; and that the upper currents may be inferred from the isobaric lines taken reversely, together with the isothermal lines taken directly. In other words, the regions of lowest pressure, with their ascending currents and relatively higher pressure at great heights as compared with surrounding regions, point out the sources or fountains from which the upper currents flow ; and the isothermals, by showing where on account of the relatively low temperatures the greater mass of the air is condensed in the lower strata of the atmosphere and sea-level pressure consequently is high, thus diminishing the pressure of the upper regions, point out the regions towards and upon which these upper currents of the atmosphere flow. The facts of the diurnal oscillations of the barometer in the different regions already discussed afford the strongest corroboration of these views.
The term " monsoon " has long been applied to the prevailing winds in southern Asia which blow approximately from S.W. from April to October, and from N.E. from November to April. The term is now, however, generally applied to those winds connected with continents which are of seasonal occurrence, or which occur regularly with the periodical return of the season. Since they are caused immediately by the different temperatures and pressures which form marked features of the climates of continents in winter and summer respectively, they are most fully developed round the coast of Asia, owing to the great extent of that continent. The monsoons of different parts of the coasts of Asia differ widely in direction from each other. Thus in winter and summer respectively they are W.N.W. and E.N.E. at the mouth of the Amur, N. and S.S.E. at Shanghai, N.E. and S.W. at Rangoon, N. and W.S.W. at Bombay, N.W. and S.W. at Jerusalem, and S.S.W. and N.N.E. at Archangel. The Indian winter monsoon generally begins to break up in March, but it is not till about the middle of May, when the normal pressure has been decidedly diminished over the heated interior, that the summer monsoon acquires its full strength and the heavy monsoonal rains fairly set in. In October, when the temperature has fallen considerably and with the falling temperature the pressure of the interior has risen, the summer monsoon begins to break up, and this season is marked by variable winds, calms, and destructive hurricanes. As the temperature continues to fall and pressure to rise, the winter monsoon again resumes its sway. Monsoons, equally with the trade-winds, play a most important part in the economy of the globe. The relatively great force and steadiness in the direction in which they blow, and the periodical change in their direction, give facility of intercourse between different countries ; and, besides, by the rainfall they bring they spread fertility over extensive regions which otherwise would be barren wastes.
The winds of Australia are also strictly monsoonal, but owing to the small extent of that continent, and consequently the smaller differences there are between the normal pressure of the interior and that of the surrounding coasts in summer and winter respectively, they are less strongly marked than are the monsoons of southern Asia ; and particularly they neither blow with the same force nor so steadily from the same point of the compass. For the same reason the Australian climates are characterized by the occurrence of more frequent droughts than are the climates of southern Asia, and the same remark applies to the climates of southern Africa.
Since the Malay archipelago lies during the summer of the northern. hemisphere between the high pressure of central Australia and the low pressure of Asia, and during the winter between the high pressure of Asia and the low pressure of central Australia, it follows that the winds of these islands are eminently monsoonal in their character, being in summer southerly and in winter northerly. The result of this peculiar wind system of the archipelago is to give to these islands a singular diversity of climates, which will be more particularly referred to under rainfall.
At Zanzibar the prevailing wind in July is S.E., but in January, when the low pressure of the interior is situated much farther to southward, it is N.E.: and the same influence is felt, though in a greatly modified degree, as far as Mauritius, where the S.E. trade changes nearly into E. during the summer. On the other side of Africa the S.E. trade of the South Atlantic is changed into a S.W. monsoon on the coast of the Gulf of Guinea.
In the southern, central, western, and northern regions of North America the prevailing winds have a well-marked monsoonal character. The prevailing winds of winter and summer respectively are N.E. and S.S.E. at New Orleans, N.W. and S.W. in Utah, N. and S. at Fort Yuma (California), E.S.E. and N.W. at Portland (Oregon), and S. and E.N.E. at Fort York, Hudson Bay. These winds are readily accounted for by the distribution of pressure over the continent in winter and summer. On the Atlantic seaboard of the United States the prevailing winds of winter vary from N.W. in the New England States to W. in South Carolina ; whereas in summer they vary generally from S.S.W. in South Carolina to S.W. in the New England States. Hence over the eastern States the summer winds are not directed towards the low-pressure region of the interior of the continent, but are determined by the relations of their pressure to the high pressure of the Atlantic to the eastward, and to the lower pressure over-spreading the Atlantic to the N.E. This influence of the Atlantic may be considered as felt westward through the States as far as the Mississippi.
Though not so decidedly marked, the winds of Europe, except the extreme south, are also monsoonal. In winter they flow from the land towards the region of low pressure in the north of the Atlantic ; but in summer the arrows, representing the prevailing winds, show that all but the extreme south of Europe is swept bywesterly winds, which flow in a vast continuous stream from the Atlantic towards the central regions of the Old Continent, and which deposit in their course the rains they have brought from the ocean.
Similarly, monsoons prevail on the coasts of Brazil, Peru, North Africa, and many other regions which happen to lie between other regions whose temperatures, and therefore pressures, differ markedly from each other at different times of the year.
These are the chief prevailing winds of the globe when the differences of the normal atmospheric pressure are such as to cause a decided and steady movement of the atmosphere over a large portion of the earth's surface, resulting in well-marked prevailing winds. But there are other winds which are greatly influenced by local causes, such as the nature of the ground, whether covered with vegetation or bare ; the physical configuration of the surface, whether level or mountainous ; and the vicinity of extensive sheets of fresh or salt water. An important characteristic of winds in their practical relations to climate is their quality, - they being warm or cold, dry or moist, according to their direction and the nature of the earth's surface over which they have just passed. Thus in the northern hemisphere southerly winds are warm and moist, while northerly winds are cold and dry. In Europe southwesterly winds are moist and easterly winds dry, while in the New England States and Canada north-easterly winds are cold and raw and north-westerly winds cold and dry.
In particular regions certain meteorological conditions occur at stated seasons intensifying these effects, resulting in excessive drought, heavy rains, intense or great heat, thus giving rise to the following among other well-known winds. The east winds of the British Islands occur chiefly in spring, but also in a less degree in November, being in the latter case often accompanied with fog. The winds here referred to are dry and parching, and their deleterious influence on the health is seen, not merely in the discomfort and uneasiness they impart to the less robust of the population, but also in the largely increased mortality which they cause from consumption and all other diseases more or less connected with the nervous system. In the countries bordering on the north of the Atlantic, atmospheric pressure reaches the annual maximum in May, and it is above the average during the other two spring months. In these months the normal pressure approaches nearer to what obtains farther south, and an examination of daily weather maps shows that this is due to the repeated occurrence in spring of very high pressures in the north of styled Arctic anticyclonic areas bring with them qualities as noxious as those of the east wind itself, and prove as injurious to health and vegetation. The cold dry wind of April 29, 1868, which blasted and shrivelled up vegetation in Scotland, particularly in the western counties, as effectually as if a scorching fire had passed across them, was a west wind.
In the south of Europe, during the winter and early spring, peculiarly dry, cold, and violent northerly winds are of occasional occurrence. Of these winds the "mistral" is one of the most notorious, which is a steady, violent, and cold north-west wind blowing from central and eastern France down on the Gulf of Lyons. It is particularly trying while it lasts to invalids who are spending the winter at the various popular sanataria which are scattered along this part of the Mediterranean coast. The great cold that took place in the north of Italy and south of France in the beginning of 1868 was a good example of the mistral. The meteorological conditions under which it occurred were unusually low pressure over the Mediterranean to southward (29.450 inches), whilst at the same time pressure rose steadily and rapidly on proceeding northward to 30'905 inches in the north of Russia. From this geographical distribution of the pressure, northerly winds swept southwards over Europe, carrying with them the low temperatures of the higher latitudes, and became still colder and drier on crossing the Alps before they made the descent on the shores of the Mediterranean. The cold tempestuous winds which descend from the Julian Alps and sweep over the Adriatic, and the dreaded "gregale " of Malta, which is a dry cold north-east wind, are in their character and origin quite analogous to the mistral.
The " northers," or " no•tes," are peculiarly dry cold strong winds which repeatedly occur from September to March in the States bordering on the Gulf of Mexico, and are perfectly analogous to the mistral. The conditions under which they occur are a pressure lower than usual to the south or south-east over the Gulf of Mexico, together with a pressure even higher than the high normal which is so marked a feature of the meteorology of the Rocky Mountains during the colder months. When, as most frequently happens, they occur in the wake of a storm, their disagreeable qualities of extreme dryness, cold, and violence arc all intensified. From a temperature of upwards of 80° experienced as the storm comes up the thermometer rapidly falls to 18° or even lower ; and, as the low temperature often occurs with a wind blowing with great violence, the northers prove most deleterious. A violent wind with a temperature of 18° is altogether unknown in the British Islands.
The "pampero " is a strong, dry, cold wind which blows across the pampas of the River Plate of South America, occurring at all seasons, but most frequently during the spring and summer from October to January. They are preceded by easterly winds, a falling pressure, a rising temperature, and increased moisture. A pampero is described by Dr D. Christison, and its appearance figured, in the Journal of the Scottish Meteorological Society, vol. v. p. 342, as seen advancing on the morning of November 28, 1867, in central Uruguay. In the early morning the wind blew rather strongly from north-east, and by and by clouds were seen moving very slowly from the west, throwing out long streamers eastwards. As they advanced, two dense and perfectly regular cloud-masses appeared in front, one behind the other, in close contact yet not intermingling, - the one being of a uniform leaden grey, while the other was as black as the smoke of a steamer. On arriving overhead, the front, though slightly wavy in appearance, was seen to be quite straight in its general direction, and the bands were of uniform breadth. They rushed forward at great speed under the other clouds without uniting with them, preserving their forms unbroken, being borne onward by an apparently irresistible force, as if composed of some solid material rather than vapour. They extended probably 50 miles in length, but as they took only a few minutes to pass their breadth was not great, and they appeared to diminish to mere lines in the distant horizon. At the instant the first cloud-band arrived overhead, the wind chopped round from north-east to north and then to south-west ; a strong cold blast at the same time seemed to fall from the leaden cloud, and continued to blow till both bands had passed. No rain or thunder occurred at this time, but in the confused rabble of clouds which followed low thunder continued to roll, and in a quarter of an hour rain fell, and for some hours thereafter wind, rain, and thunder continued, but only to a moderate degree. The low temperature and rising barometer and change of wind are the constant and most striking characteristics of the pampero. On one occasion the temperature fell 44° in fourteen hours, and on another occasion the fall was only 4°. Rain is a usual accompaniment, but on rare occasions the pamper() passes off and no rain falls.
Rainfall. - Whatever tends to lower the temperature of the air below the dew-point is a cause of rain. It is therefore to the winds we must chiefly look for an explanation of the rainfall, and the broad principles of the connexion may be stated to be these five : - (1) when the winds have previously traversed a considerable extent of ocean, the rainfall is moderately large ; (2) if the winds advance at the same time into colder regions, the rainfall is largely increased, because the temperature is sooner reduced below the point of saturation ; (3) if the winds, though arriving from the ocean, have not traversed a considerable extent of it, the rainfall is not large; (4) if the winds, even though having traversed a large extent of ocean, yet on arriving at the land proceed into lower latitudes or regions markedly warmer, the rainfall is small or nil ; (5) if a range of mountains lies across the onward path of the winds, the rainfall is largely increased on the side facing the winds, and reduced over the regions on the other side of the range. The reason here is that, the air on the windward side of the ridge being suddenly raised to a greater height in crossing the range, the temperature is further reduced by mere expansion, and a more copious precipitation is the result ; whereas on the leeward side as the air descends to lower levels it becomes gradually drier, and accordingly the rainfall rapidly diminishes with the descent.
We have drawn attention to the diminished velocity of the wind over land as compared with the open sea (p. 125). From this it follows that an envelope of stiller air or air of less velocity than that of the prevailing wind broods over the land, and by its presence forces the prevailing wind to a greater height, thus tending to increase the rainfall. If the foreshore rises within a few miles to a height of 200 or 300 feet, the result is very striking when the wind from the sea blows straight upon it. Thus at Spittal, near Berwick, on September 1877, a N.E. wind blew straight ashore at an estimated velocity of 25 miles an hour. To eastward the sky was singularly clear down to the horizon, but to westward all the country beyond a mile from the shore was enveloped in what appeared a dense mist or fog. About 15° to eastward of the zenith of an observer on the shore, the thinnest rack of eloudlets was seen emerging without intermission from the deep stainless blue of the sky, which as they drifted landward increased so rapidly in volume and density that the zenith was three-fourths covered with clouds. A similar phenomenon was seen in September 1879 on board the Orkney steamer at the magnificent cliff of Hoy Island, Orkney. A heavy storm had just cleared away, and a strong W.N.W. wind was blowing right against the cliff. The sky was absolutely cloudless all round, except the upper 300 feet of Hoy Hill, 1570 feet high, which was enveloped in a thick mist that stretched away to windward, some distance to westward of the steamer's course, which was about 2 miles from land. The western termination of the cloud was the thinnest rack of cloud, which emerged unceasingly from the blue sky at a distance not less than 4 miles to windward of the cliff. The constituent parts of the cloud itself were in rapid motion eastward, but, owing to the fresh accessions it was constantly receiving, the cloud itself appeared stationary. Thus the wind was forced upward into the atmosphere for some considerable distance to windward of the ridge lying across its path.
It is this dragging effect of the land on the wind, and the consequences which result from it, that explain how it is that during storms of wind and rain from the north-east the rainfall over the foreshores of the Firth of Forth, the Moray Firth, and the Pentland Firth looking to the north-east is so much in excess as compared with the rest of Scotland. The same principle explains the heavy rainfall in plains at some distance from the range of hills lying across the wind's path and on the side of the rain-bringing winds.
For short intervals of time the heaviest rainfalls occur with tornadoes, waterspouts, and some other forms of the whirlwind, the reason being that not only is there rapid Seathwaite, Cumberland, November 27, 1848; and 7.12 inches at Drishaig, Argyllshire, December 7 to 8, 1863. But it is in lower latitudes that the heaviest single showers have been recorded. The following are among the most remarkable : - at Joyeuse, France, 31.17 inches in twenty-two hours; at Genoa, 30.00 inches in twenty-four hours; at Gibraltar, 33.00 inches in twenty-six hours; on the hills above Bombay, 24-00 inches in one night ; and on the Khasi Hills, India, 30.00 inches on each of five successive days.
As regards the ocean, there are no available data from which an estimate could be formed as to the amount of the rainfall, since the rainfall statistics of the ocean must be regarded as giving hardly anything more than the comparative frequency of the fall. It is, however, certain that the equatorial belt of calms in the Atlantic and Pacific between the trades is the region where the ocean rainfall reaches the maximum, and the parts of these oceans are the rainiest which are the longest within the belt of calms as it shifts its position northward and southward with season. While the cloud-screen is undoubtedly dense, and the rainfall frequent and heavy, the careful observations of the " Challenger " and "Novara " show that the statements generally made as to these points are greatly exaggerated.
In the regions of the trades the rainfall is everywhere small over the open sea, seeing that the trade-winds are essentially an outflow from anticyclonic regions, and their original dryness is to a large extent maintained because their course is directed into regions which become constantly warmer. Thus at Ascension, lat. 8° 45' S., which is throughout the whole year within the S.E. trades, the mean rainfall for the two years 1854-55 was only 8.85 inches. At St Helena, which lies constantly within the same trades, five years give a mean rainfall of 5.36 inches on the coast ; but in the same island at a height of 1763 feet the annual amount rises to 23.98 inches. Malden Island and some other islands in the Pacific, about long. 150° W., and for some distance on each side of the equator, have been pointed to by Scott as practically almost rainless, as is shown by their containing extensive guano deposits. These islands are situated somewhat similarly to Ascension with respect to the zone of calms. In Mauritius the annual rainfall on a mean of four years was 30 inches at Gros Cailloux, but at Cluny, only 16 miles distant, for the same four years it was 146 inches ; in regard to which Meldrum remarks that at Cluny, which is in the vicinity of mountains and forests, in the southeast of the island, and thus directly exposed to the trade-wind as it arrives from the sea, the rainfall in almost any month is from four to six times greater than at Gros Cailloux on the north-west coast, where neither mountain nor forest exists, and where the S.E. trade arrives considerably drained of its moisture.
From what has been said it is evident that the heaviest rains will be brought by the winds which have traversed the greatest extent of ocean within the tropics, and which accordingly of all ocean winds have the highest temperature and humidity. These conditions are most completely fulfilled during the summer months of the northern hemisphere by the winds which, commencing from near lat. 30° S., blow home on southern Asia as the well-known S.W. monsoon of these regions. Accordingly it is by the winds of this monsoon that a larger rainfall is distributed over a larger portion of the earth's surface than occurs anywhere else in any season ; and this large rainfall is in many regions still farther greatly increased by the mountain ranges which lie across the path of the rain-bringing winds.
It is on these winds that the rainfall of India chiefly depends. Along the whole of the west coast from the Gulf of Cambay southward, and on the Western Ghats, the rainfall is excessive. The following are some of the more interesting annual means in inches beginning with Bombay and proceeding southwards : - Bombay, 74 ; Matlierarn, 247 ; Mahabaleshwar, 252 ; Ratnagiri, 104; Laura, 255 ; Goa, 102 ; Karwar, 115 ; Honawar, 139 ; Mangalore, 134 ; Cannanore, 132 ; Calicut, 116 ; and Cochin, 114. In the west of Ceylon the rainfall is also heavy, being at Colombo 87, at Galle 91, and at Ratnapura, at some distance inland among the hills, 149. Since the S.W. monsoon is drained of much of its moisture in crossing these mountains, a greatly diminished rainfall is distributed over the interior and east side of India, and on the eastern slopes of Ceylon.
If now we cross to the eastern shores of the Bay of Bengal, we again encounter an excessive rainfall along these coasts and up the slopes of the mountains looking down on them. Thus from south northward the following are among the more characteristic rainfalls in inches : - Nancowry, 102 ; Port Blair, 116 ; Mergui, 152 ; Tavoy, 196 ; Maulinain, 189 ; Rangoon, 100 ; Bassein, 98 ; Sandoway, 212 ; Akyab, 198; and Chittagong 104. On the other hand, at Thyetmio, inland on the Irawadi, the annual rainfall is only 48 inches.
We have shown how, in accordance with the peculiar distribution of pressure in India in summer, the monsoon is diverted up the valley of the Ganges as an E.S.E. wind, distributing on its way, even to the head of the valley, in a generous rainfall the moisture it has brought from the Indian Ocean and the Bay of Bengal. The rainfall does not extend farther westward than the basin of the Ganges, and the precipitation is most copious along the lower Himalayas, the largest falls being recorded at heights about 4000 feet, - being, as pointed out by Hill, near the level at which the summer monsoon is cooled just below its dew-point. The following are some of the larger rainfalls in inches, beginning with the more western : - Mussooree, 95 ; Naini Tal, 92 ; Khatmandu, 57 ; Darjiling, 121 ; Kurseong, 154 ; Buxa, 219 ; Kuch Behar, 131.
The rainfall is very large in the north-east angle of the Bay of Bengal and thence northwards towards Bhutan, or at the angle where the summer monsoon from the bay curves round to a westerly course on its way up the Ganges. Thus at Noakhally, on the coast, it amounts in inches to 109 ; at Tura, on the Brahmaputra, immediately to west .of the Garo Hills, 129 ; at Silchar and Sylhet to eastward, 117 and 155 ; whilst at Cherrapunji, on the Khasi Hills, it rises to 493.19 inches on a mean of twenty-four years. This last rainfall is the largest known on the globe, the causes of which are the highly saturated state of the monsoon on its arrival at the lower Ganges, the high mountain range of Burmah to eastward of Bengal, which turns the monsoon to the north, and the protrusion westwards of the Khasi and Garo Hills so as to lie in the line of that branch of the monsoon which passes from the lower Ganges into the basin of the Brahmaputra above Goalpara. The consequence is that the highly saturated air of the monsoon in its passage across the Khasi Hills is suddenly raised to a height of about 6000 feet, and being thereby reduced far below the point of saturation the superabundant moisture is precipitated in unequalled deluges of rain. The amount of the annual rainfall at all these places is determined, essentially if not altogether, by the rains of the summer monsoon, the relative intensity of which over India may be taken to be fairly represented by the rainfall of July.
The rains which accompany the N.E. monsoon of the winter months may be represented by the rainfall for January. These are heaviest in Ceylon, especially on its east- slopes, and in southern India, or where the N.E.
monsoon arrives after having traversed a large extent of ocean. The fall for the month exceeds 6 inches over a large portion of the east coast, whilst at Colombo in the west the rainfall is only half that amount, and farther north at Pattalum the January rainfall is only 1•82 inches. In southern India the amount varies from about 1 to 2 inches. Blanford pointed out in 1873 (Phil. Trans., vol. clxiv. p. 618) that, while the surface winds of northern India in winter are northerly, on the Himalayas, especially the northwest portion, southerly winds prevail during the cold months. It is these upper southerly winds which bring the winter rains to the Punjab, Upper India, and the highlands of Assam. It is further to be noted that winter rains also occur in Central India, where the prevailing surface winds are from east and north-east. The mean rainfall of January at Mussooree is 2.00 inches and at Naini Tal 2.86 inches, and in Assam, at Sibsagar, 1.13 inch. Over a large tract of the east side of southern India from Nellore southward, including Ceylon, the maximum rainfall for the year occurs in the months of October and November.
Rainfall of the Malay Archipelago and Australia. - Under the direction of the late Dr I3ergsma, systematic observations of the rainfall of the Malay archipelago were begun in 1879, the number of stations being 150. The results of the first time years show that the mean annual rainfall over the archipelago varies from about 60 inches in Timor to upwards of 200 inches at some spots among the western slopes of Sumatra. But the most important feature in the rainfall in its relations to climate is not the absolute amount that falls annually, but rather the manner of its distribution through the months of the year. Over the greater number of the islands rain falls copiously every month; but as regards some of the islands the year is divided into dry and wet seasons as marked as are seen in the climates of India. The key to this essential difference among the climates is the distribution of atmospheric pressure during the months of the year from south-eastern Asia to Australia, with the resulting prevailing winds. During the winter months atmospheric pressure is high in south-eastern Asia and low in the interior of Australia, the difference being about three-quarters of an inch. Since between these two regions the fall in the mean pressure is practically uninterrupted, the Malay archipelago lying between them is swept by northerly winds (fig. 14). As these winds have traversed a great breadth of ocean in their course, they arrive in a highly saturated state, and consequently deposit a copious rainfall, particularly on the northern slopes of the higher islands. Hence in these months the rainfall over the islands without exception is large, the mean monthly amount being in many cases more than 30 inches. These winds continue their course to southward towards the low-pressure region in the interior of Australia, and deposit along the north coasts of that continent a monthly rainfall rising generally to from 14 to 20 inches. On advancing into the interior, the mean amount gradually diminishes at the successive telegraphic stations to 3.50 inches at Alice Springs near the tropic of Capricorn. The amount of the rainfall for any particular year, and the distance from the coast to which the rains penetrate inland, depend essentially on the height of the winter pressure of south-eastern Asia as compared with the low mean pressure of central Australia, by which the strength of the northerly monsoon is regulated.
On the other hand, during the summer of the northern hemisphere pressure is high in the interior of Australia and low in China, the mean difference being about half an inch. Between the two regions the fall in the mean pressure is continuous and uninterrupted, and as a consequence southerly winds prevail over the intervening archipelago. These winds, as they advance from the continent into lower latitudes, are absolutely rainless in the north of Australia, and over Timor and the other Malay islands which are separated from Australia only by a comparatively narrow belt of sea. During the three years no rain whatever fell in Timor in July and A igust, and the fall in June, September, and October was small. As, however, the winds pursue their course to northward, they tagerly lick up moisture from the sea, so that by the time they arrive at Amboyna they have become so saturated that the monthly ram fall there rises to nearly 30 inches. Again at some distance to the west of Timor rain falls more or less regularly every month, the amount increasing in proportion to the extent of ocean traversed by the S. E. winds, which advance towards these islands from the direction of Australia. These marked differences among the climates of the Malay archipelago, which, since they really depend on the geographical distribution of land and sea of this part of the globe, must be regarded as permanent differences, have played no inconspicuous part in the singular distribution of animal and vegetable life which characterizes the archipelago.
In July the prevailing wind in West Australia is N.W., and the rainfall reaches the maximum for the year, whereas in January the wind is S. E., and the rainfall is the minimum. Similarly in January since the winds of the southern half of South Australia and Victoria are from the south, and thus blow towards warmer regions, the rainfall is either at the annual minimum, or it is small. But on rounding the coast and proceeding northward, the wind becomes E., then N.E., and ultimately N. in the north of Queensland. With this prevalence of oceanic and equatorial winds, the rainfall at this time of the year rapidly rises over the whole of the eastern slopes, till at Cape York it is about 20 inches. In the basins of the Murray and Darling rivers, which are shut off from the east by the mountain ranges of New South Wales, the rainfall is only about an inch and a half. On the other hand, to south of the latitude of Sydney, including Tasmania, the maximum rainfall occurs in winter over those regions which slope south towards the sea. On crossing the mountain range of Victoria into the basin of the Murray river, the rainfall rapidly diminishes. In the north of New Zealand the winter rainfall is the heaviest ; but farther south, where westerly winds prevail with some steadiness through the year, the rainfall is more equally distributed through the months ; and, as the prevailing winds are westerly, the heaviest rainfall is in the west of the islands. Thus at Hokitika in the west near sea-level, and not far from a lofty range of mountains to the east, the annual amount reaches 120 inches, and at Bealey inland at a height of 2104 feet it is 106 inches. At Wellington the annual rainfall is 52 inches, at Southland 46, at Dunedin 34, and at Christchurch 25, thus showing, in the rainfall of the two sides of the island, extremes nearly as great as in Scotland.
Rainfall of Europe. - As regards rainfall, Europe may be conveniently divided into two distinct regions, - western and northern Europe, extending in a modified degree through the interior of the continent into Siberia, and the countries bordering on the Mediterranean. A vast ocean on the one hand, a great continent on the other, and a predominance of westerly winds are the determining circumstances in the distribution of the rainfall over western Europe. Hence the rainiest regions are to be found in the west, where mountain ranges stretch north and south. The annual rainfall exceeds 80 inches over a considerable district, including the greater part of Skye and portions of the counties of Inverness and Argyll to the south-east, in the lake district of England, and in the more mountainous parts of North Wales, - these three districts being the wettest in Europe. As Ireland presents no continuous range of mountains opposing the westerly winds of the Atlantic, no Irish rain-gauge shows a mean rainfall of 80 inches. A point of some interest is suggested by the rainfall of the counties of Kirkcudbright and Dumfries in Scotland. These counties offer to the westerly winds a series of valleys sloping south to the Solway Firth, which show successively a diminished rainfall on advancing eastward till at several places in Nithsdale and Annandale it does not exceed 40 inches. But in Eskdale, farther to the ( ast, the rainfall instead of falling increases to about 60 inches. The reason is that the westerly winds are obstructed in their onward course by the range of hills by which Eskdale is bounded on the east, in surmounting which the winds are much reduced in temperature, and their superabundant moisture falls in copious rains immediately to westward of the ridge. The cause of the larger rainfall of Eskdale is thus analogous to that of the large rainfall of the coast in the north-east of the Bay of Bengal immediately under the Assam range of mountains, In England the largest annual rainfall is 146 inches at Seathwaite in the Lake district, in Scotland 128 inches at Glencroe in Argyll, whilst in Ireland the largest is only 76 inches. The driest part of the British Islands is an extensive district to south-south-west of the Wash, with a rainfall of about 21 inches. A large extent of England, and all the more important agricultural districts in Scotland, have a rainfall under 30 inches ; the greater part of England, and nearly the half of Scotland, have a rainfall not exceeding 40 inches ; but in Ireland it is isolated patches only that show a rainfall less than 40 inches.
In the west of Norway the rainfall in inches is 72 at Bergen, 51 at Aalesund, 46 at the Naze and in the Lofoten Isles, falling to 10 at the North Cape. At Christiania, Upsala, and a large part of the east of Scandinavia the rainfall is about 21 inches, falling to 16 inches on the north coast of the Gulf of Bothnia. In Russia and Siberia it rises only at a few places to 20 inches, several districts of this extensive region having an annual rainfall of 10, 5, 3, or even 2 inches. The rainfall of Spain presents great extremes - from 68 inches at Santiago to 13 inches at Saragossa. In France and the plains of Germany the average varies from 35 to 20 inches, but in mountainous regions these figures are greatly exceeded, rising through all gradations to upwards of 100 inches at some points in the Alps.
An important distinction between the manner of distribution of the rainfall in the west of Europe and at more inland places is that the greater part of the annual quantity of the west falls in winter, whilst in the interior the amount in summer is greater than in winter. The rainfall of January and July shows this in a very forcible manner. The summer climates of the extreme south of Europe and North Africa are rainless, and over extensive regions in the south of Europe adjoining the July rainfall does not amount to an inch. Over these dry regions the prevailing winds of summer are northerly, and hence the drought which characterizes them. On the other hand, the rainfall in the interior of the continent is large. In January the maximum rainfall occurs on the mountains and high grounds overlooking the Atlantic, and the minimum on the plains of Russia.
Owing to the way in which Europe is broken up by the seas which diversify its surface, the time of the year when the rain attains the maximum differs greatly in different regions. This phase of the rainfall occurs, indeed, according to locality, in all months except February, March, and April. The month of occurrence of the annual maximum rainfall over Europe is shown by fig. 18. A similar map representing the month of least rainfall shows still greater uniformity in a regular succession of the months in passing from region to region. Thus the month of least rainfall is January on the lower Volga, February in western Russia and the greater part of central Europe, March in the north of France and south of Great Britain, April farther to the north, May in Scotland north of the Grampians, June in Orkney, Shetland, Iceland, the west of Ireland, and the north-west of France, and July over the whole of the south of Europe. The driest month occurs nowhere in Europe in any of the five months from August to December.
Rainfall of North America. - West of the Rocky Mountains the rainfall is very unequally distributed, the annual amounts varying from 86 inches at Astoria, near the mouth of the Columbia river, to 8 inches at San Diego on the coast, and 3 inches at the head of the Gulf of California. Over the whole of the region between the Cascade and Rocky Mountains the rainfall at all seasons is extremely small, this being indeed that feature in the climate to which the formation of the canons of that region is chiefly to be referred. On the other hand, in the United States and Canada to east of long. 100° W. the distinguishing feature of the rainfall is the comparative equableness of its distribution, an annual rainfall exceeding 50 inches occurring only over restricted districts, and a rainfall as low as 20 inches being scarcely met with anywhere. The regions where the rainfall exceeds 50 inches arc Florida, the lower basin of the Mississippi, and the Atlantic seaboards of Nova Scotia and Newfoundland.
In January the annual maximum rainfall occurs over the whole of the west coast from Sitka to lower California; but in the interior between long. 120° and 95° W. the amount is everywhere small, and over a considerable part in the south-west of this region no rain falls. The region of largest rainfall extends from Louisiana to West Virginia, where the mean varies from 4 to 6 inches. Over nearly the whole of the Dominion of Canada, by much the greater part of the winter precipitation is in the form of snow, which has been carefully measured and recorded by the Meteorological Service. The average snowfall for January exceeds 30 inches at St John's, Newfoundland, in Anticosti, Prince Edward Island, and in many other regions.
In July the rainfall is everywhere small in the west, a large part of this extensive region being absolutely rainless. The remarkable dryness of the climate at this season is due to the N.W. winds that set in towards the low pressure of the interior, which thus blow towards warmer regions. The rainfall to the east of the Rocky Mountains is distributed by the winds which are connected with the low-pressure region of the interior and with the high-pressure region of the Atlantic. The result is two regions of larger rainfall, the one in the south-east of the States and the other to the west of the lakes. The summer winds of the south-eastern coasts are southerly, and as they are anticyclonic in their origin and have in their course traversed some extent of ocean, they arrive well- but not super-saturated, and pour down a rainfall in July of 6 inches and upwards along the coasts and for some distance inland from Louisiana to Chesapeake Bay. Further, since in July these winds attain their maximum force and persistency, the rainfall at the same time reaches the maximum along the whole coast from Boston to some distance west of New Orleans. Since the summer winds blow in the line of the Allegheny mountains and not across them, the rainfall diminishes in ascending their slopes. The comparative equableness of the rainfall over the eastern States is the necessary result of the winds' passing into higher latitudes, and, therefore, cooler regions. A broad region where the rainfall is less than on each side of it, extends from Michigan to the south-west as far as Canadian River. To the west of the lakes the rainfall rises above 4 inches, and, since over this region the winds become somewhat easterly as they flow towards the low-pressure area, it is probable that the larger rainfall of this prairie region has its origin in no small degree in the evaporation of the lakes. On ascending the higher reaches of the Mississippi, the amount diminishes, but scarcely falls lower than 2 inches, being thus analogous to the summer rains of the Upper Ganges. On crossing the water-parting into the basin which drains into Hudson Bay, we encounter E. and N.E. winds laden with vapour licked up in their passage over Hudson's Bay, which they distribute in a generous rainfall of probably 3 to 5 inches over the rising colonies of Manitoba and Saskatchewan. An important point in the climate of the States is that over nearly the whole of the extensive region stretching between Alleghenies and Rocky Mountains, except the south coast already referred to, the annual maximum rainfall does not occur in summer but in spring, the month of largest rainfall in the great majority of eases being May. In the basin of Hudson's Bay July is the month of largest rainfall.
Rainfall of Central and South America. - The following are, in inches, the larger and more interesting annual rainfalls round the coasts : - Vera Cruz, 182; Belize, 75 ; Maracaibo, 163; Caracas, 155; Georgetown, 95 ; Paramaribo, 142; Cayenne, 140 ; Para, 71; Pernambuco, 109 ; Buenos Ayres, 34; Bahia Blanca, 19 ; Puerto Montt, 102 ; Valdivia, 109; Valparaiso, 100; Serena, 93 ; Lima, 9; and a large part of Peru, nil. A remarkable feature of the rainfall of South America is the large amounts that fall in the basins of the Orinoco and Amazon ; the fall is 91 inches in the upper basin of the Madeira, and 112 inches at Yquitos (lat. 3° 40' S., long. 72° 57' W.). The reason is that this immense region, where pressure appears to be almost constantly low, is open to the highly saturated winds that blow from the equatorial Atlantic. Quite different is the distribution of the rainfall over the La Plata basin. The annual falls, in inches, are 92 at Joinville, 58 at Corrientes, 44 at Monte Video, 36 at Parana, 24 at Santiago, 22 at San Luis, and only 6 at Mendoza. The fall rapidly rises in ascending the eastern slopes of the Brazil mountains facing the South Atlantic ; thus, while the amount at Rio Janeiro is 45 inches, on the hills to northward it is 116 inches.
In January northerly winds prevail on the south coasts of the Gulf of Mexico and the Caribbean Sea, and as they have their origin in the high pressure of the American continent, and in crossing the sea pass into lower latitudes, the January rainfall of these coasts is comparatively small. In July, however, the prevailing winds are easterly, and as they have traversed a large extent of the equatorial waters of the Atlantic they are highly saturated, and consequently the July rainfall of these coasts is everywhere very large. The following are, in inches, the January and July rainfalls : - Caracas, 1'00 and 14'04 ; Guatemala, 0.28 and 10'79 ; Vera Cruz, 5-10 and 35.90. The seasonal distribution of the rainfall in the basin of the Amazon is the reverse of this. In January the position of the belt of calms is about lat. 3° N., and as pressure is relatively low over the basin of the Amazon, especially its southern slopes, the trades and the west portion of the region of calms unitedly spread their highly saturated air over the whole region as far as the Andes, resulting in one of the most widespread heavy rainfalls anywhere to be met with. On the other hand, since in July the belt of calms is about lat. 10° N., the saturated atmosphere of the tropical regions no longer flows up the Amazon, hut is carried westward into the Caribbean Sea and Gulf of Mexico. Hence at this season the rainfall of the Amazon valley is small. The following are, in inches, the January and July falls : - Para, 6'51 and 3.26 ; Manus, 7'33 and 1.82 ; upper Madeira, 15.90 and 0.30 ; and Yquitos, 10'24 and 4'26. On the La Plata in January pressure is low, and as winds consequently blow from the ocean in upon the region of low pressure the rainfall is large ; but as pressure is high in the interior in July the rainfall in that month is small. The following are, in inches, the January and July rainfalls : - Buenos Ayres, 2'37 and 1.70 ; Parana, 4'63 and 1'32 ; Corrientes, 5.24 and 2'67 ; Joinville, 14.26 and 3'55 ; and San Luis, 2.63 and 0'00.
Rainfall of Africa. - As regards the rainfall, Africa presents the greatest diversity in its climates. The following are the annual amounts in inches at various points on or near the coast : - Port Said, 2; Alexandria, 8; Tunis, 12; Algiers 31; Oran, 17; Mogador, 50; mouth of the Senegal, 17; Goree, 21; Sierra Leone, 126; Christiansborg, 23; St Thomas, 40; Gaboon, 106; Loanda, 11; Cape Town, 23; Mossel Bay, 12; Port Elizabeth, 24; Durban, 43; Zanzibar, 58; and mouth of the Zambezi, 61. In the north of the continent, the rainfall rapidly diminishes inland, and over the great desert of Sahara practically none falls. In the interior of Algiers it diminishes, the amount at Laghouat being 17 inches, and at Biskra 9. In Egypt the rainfall is limited to a narrow strip along the coast ; at Cairo the annual fall scarcely amounts to an inch. The January and July rainfalls are, in inches, as follows : - Port Said, 0'46 and 0.00; Alexandria, 1.95 and 0'20; Algiers, 4.43 and 0'04; Biskra, 0'56 and 0'03; St Louis (Senegal), 0.28 and 3'00; Goree, 0'00 and 4'06; Sierra Leone, 0.69 and 24.20; Christiansborg, 0.50 and 2-00; Katunga, 0.11 and 4'76; Gaboon, 9.35 and 0.48; Cape Town, 0.28 and 3.83; Durban, 5'00 and 1.70; Pretoria, 6'07 and 0'71; and Zanzibar, 2.02 and 2.35. At Zanzibar the heaviest rains occur about the equinoxes, the mean for April being 14'55 inches, and for October 6.80 inches.
In the ease of this, as the other continents, the explanation of the different amounts is to be had in the seasonal changes of wind. In the north the winter rains are to a very large extent the accompaniment of the Mediterranean storms of that season, but in summer pressure is diminished in the interior and increased in the Atlantic to the north-west, resulting in strong steady northerly winds, which as they advance into hotter regions are unaccompanied with rain. The heavy summer rains from Senegambia to the Gold Coast are due to the strong monsoonal winds which set in towards the interior, thus drawing over these coasts the highly saturated air of the belt of calms and of the trades immediately to the north and south of it. Since in winter the belt of calms is removed 8° of latitude farther to the smith, and the temperature of the interior is greatly reduced, it follows that the winds blowing on these coasts from the sea are drier and less strong, and consequently the rainfall is small. At Sierre Leone the absolutely driest month is February, 0.31 inch, and the wettest September, 29.15 inches. On the other hand, at Gaboon (lat. 0* 25 N.) the dry season is from June to August, when the belt of calms is farthest to the north ; and time absolutely rainiest about the equinoxes, the mean of March being 14.70 inches and October 19'52 inches. At Loanda (lat. 8' 49' S.) the annual amount is only a tenth of what falls at Gaboon, and it falls wholly during the summer months of the sonthern'hemisphere. In South Africa pressure in January is lowest in the interior, towards which prevailing winds from the ocean blow, and as these advance into regions becoming rapidly hotter the rainfall all round the coast and for some distance inland falls to the annual minimum. But in more strictly inland districts which are at a considerable elevation the rainfall reaches the maximum at the same season. Thus the amounts in inches for January and July are - for Pretoria, 6-07 and 0.71; Maritzburg, 4.23 and 0-21; Graham's Town, 2•89 and 1.51; Lower Net's Poort, 1.33 and 0.49; and Aliwal North, 1.55 and 0-00. In the winter months pressure in the interior is high, and the rainfall consequently small. Though on the coast winds from the arid interior frequently prevail, yet the storms that sweep eastward past South Africa precipitate over large portions of the southel.n slopes of this part of the globe what must in the main be regarded as a generous rainfall. It follows that the climates of these important colonies range themselves into two perfectly distinct classes, - the climates of the inland regions and the Natal coast, where the rains occur during the hottest months, and the climates of the other regions, where the annual rains occur during the coldest months. Little is accurately known regarding the rainfall of the interior of Africa. It is certain, however, that it is small, or nil, over the extensive region of the Sahara, and that it is large front about 15° N. lat. to some distance south of the equator. Probably the rainiest part of Africa is the region extending from the Victoria Nyanza northwards to and including the gathering grounds of the two great tributaries of the Nile.
Snow. - Snow takes the place of rain when the temperature is sufficiently low to freeze the condensed moisture in the atmosphere. Snow is composed of crystals, either six-pointed stars or hexagonal plates, which exhibit the greatest variety of beautiful forms, one thousand different kinds having been observed. These numerous forms Scoresby reduced to five principal varieties : - (1) thin plates, comprising several hundred forms of the most exquisite beauty ; (2) a nucleus or plane figure, studded with needle-shaped crystals; (3) six-sided, more rarely three-sided, crystals ; (4) pyramids of six sides ; (5) prismatic crystals, having at the ends and middle thin plates perpendicular to their length. In the same snowfall the forms of the crystals are generally similar. The flakes vary from 0.07 inch to an inch in diameter, the smallest occurring with low temperatures and the largest when the temperature approaches 32°. If the temperature is a little higher, the snow-flakes are partially thawed in falling through it, and fall as sleet. The white colour of snow is caused by the combination of the different prismatic colours of the minute snow-crystals. The density of snow is far from uniform ; it is generally from ten to twelve times lighter than an equal bulk of water, but varies from eight to sixteen times lighter than water.
The limit of the fall of snow near sea-level coincides roughly with the winter isothermal of 52°, since in places where the mean winter temperature is no higher than 52° that of the air fulls occasionally to 32° or lower during the winter months. As regards Europe, the southern limit is about Gibraltar; in North America it is Savannah, New Orleans, the mouth of the Rio Grande, the head of the Gulf of California, and San Francisco. In Europe, north of lat. 60°, snow falls generally on an average of from so. to 110 days in the year. At Upsala the number of days is 61, at Warsaw 45, Aberdeen 42, Oxford 18, Ostend 15, Brussels 27, Tarum (in the south-west of Jutland) 12, Copenhagen 23, Vienna 33, Odessa 19, Sebastopol 12, Milan 11, Trieste 6, Saragossa 5, Madrid 3, and Lisbon 1. In Greenland the number of days exceeds 80, and this figure is nearly reached in Newfoundland and the northeast seaboard of Nova Scotia. At Quebec the mean days of snow are 66, Halifax 64, Winnipeg 54, Detroit 34, Cape Henry 13, St Louis 11, mouth of the Columbia River 7, and Charleston 2. In Russia the time of the year when snow falls most frequently is December and January, except in the south of the empire, where February is the month of the most frequent occurrence of snow. But to the north of a line drawn from the entrance of the Gulf of Finland through Warsaw, Cracow, Salzburg, and Santiago March is the month of maximum occurrence in the great majority of instances ; while to the south of this line it is January and in several cases December.
The largest falls of snow occur in the Antarctic regions, as is well attested by the magnificent icebergs of solidified snow which break off all round from the lofty walls of ice that engirdle the Southern Ocean. Excepting perhaps in the Dominion of Canada, no data have been anywhere collected from which even a rough estimate could be formed as to the mean annual amount of snow that falls in different parts of the globe.
Snow-Line. - The snow-line marks the height below which all the snow that falls annually melts during summer. No general rule can be stated for this height in different climates owing to the many causes determining it. These are the exposure of mountain slope to the sun (and hence, other things being the same, it is higher on the south than on the north sides of mountains), exposure to the rain-bringing winds, the steepness of the mountains, and the degree of dryness of the air. Hence the position of the snow-line can be known by observation only. It falls only little on either side of the equator to lat. 20°; from lat. 20° to 70° it falls equably, but from lat. 70° to 78° much more rapidly. To this general rule there are many exceptions. It is 4000 feet higher on the north than the south side of the Himalayas, owing to the larger snowfall on the south, and the greater dryness of the climate of the north side, and therefore the greater evaporation from the snow there. It is higher in the interior of continents than near the coasts, because the precipitation is less and summer heat greater. In the Caucasus it is 11,063 feet high, but only 8950 in the Pyrenees. In South America it rises from the equator to lat. 18°, and more on the west than on the east slopes of the Cordilleras, owing to the large precipitation on the east and small precipitation and arid climate of the west side of that chain of mountains. It is as high in lat. 33° S. as in 19° N., but south of that latitude it rapidly sinks owing to the heavy rains brought by the moist N.W. winds of these regions. In the south of Chili it is 3000 feet lower than in the same latitudes in Europe, and 6000 feet lower than in the extremely arid climates of the Rocky Mountains.
Storms. - If weather charts representing a large part of the northern hemisphere be examined, two distinct systems of pressure are seen which change their forms and positions on the earth's surface from day to day. The one set are systems of low pressure marked off by concentric isobars enclosing pressures successively lower till the centre is approached ; and the other systems of high pressure marked off by concentric isobars enclosing pressures becom ing successively higher towards the centre. The former of these are called cyclones, and the latter anticyclones. These areas of low pressure are the distinguishing characteristics of the hurricanes and typhoons of tropical regions, and of the ordinary storms of higher latitudes, and they may all be conveniently grouped under the general name of cyclones. Fig. 19 shows a storm which was passing across northwestern Europe on the morning of November 2, 1863, and it may be taken as fairly representing the general features of cyclones. In the figure the arrows fly with the wind, and the force of the wind is indicated by the number of feathers on the arrows.
It will be seen that the winds indicate, not a circular movement round the centre of lowest pressure, hut a vorticose notion inwards upon that centre, the motion being opposite to that of watch-hands. In other words, the wind follows Buys Ballot's law, already explained. The winds are strongest where the isobars are closest together ; or they are generally proportioned to the " barometric gradicnt," - a term introduced by Stevenson in 1867. Cyclones have diameters seldom less than 600, and they occasionally exceed 3000 miles ; the cyclone of fig. 19 had a diameter of about 1200 miles. The cyclones of the Mediterranean are usually of smaller dimensions than those of northwestern Europe and America. The rates at which cyclones advance over the earth's surface vary greatly, the average in America being 24 miles an hour, in the Atlantic 20 miles, and in Europe 26 miles. A rate as high as 70 miles an hour has occurred in the British Islands ; sometimes they remain stationary, and more rarely their course is for a time retrograde. The temperature and humidity increase at those places towards and over which the front part of the storm is advancing, and fall at those places over which the front part of the storm has already passed. In other words, the temperature and humidity rise as pressure falls and fall as pressure rises. This is the important climatic significance of cyclones. Thus a succession of low pressures passing eastwards in courses lying to northward of the British Islands are the essential conditions of open winters ; whereas, if the cyclones follow courses lying to southward, the winters are severe. In a cyclone the broadest feature of weather is an area of rain about or rather somewhat in front of the centre, surrounded by a ring of cloud, outside which the sky is clear. The precise form and position of these areas have been shown by Abercrombie to vary with the type of pressure distribution, with the intensity of the cyclone, and with the rate of its progress, and they are also influenced by local, diurnal, and seasonal variations.
The chief point of difference between American and European storms is essentially the result of the mean winter pressures to the west and north-west of their respective storm-tracks. Owing to the high winter pressure in the interior of America, the barometer rises in the wake of the storms of the United States more rapidly, the wind veers round more quickly and more uniformly to N.W., N.N.W., and N. and keeps longer in these directions, and the temperature and humidity fall to a greater degree, than happens in Europe. In the New England States and Canada the easterly winds of the storms, coming as they do from the Atlantic, are disagreeably cold, damp, and misty in a degree and with a frequency much greater than occurs with the same winds in the British Islands.
The chief points of difference between the hurricanes and typhoons of the tropics and the cyclones of higher latitudes are these : - tropical cyclones are of smaller dimensions, show steeper barometric gradients and therefore stronger winds, and advance at a slower rate over the earth's surface. Another point of difference is that a large number of the hurricanes of the West Indies and the typhoons of eastern Asia first pursue a westerly course, which gradually becomes north-westerly, and on arriving at about lat. 30° they recurve and thereafter pursue a course to north-eastwards. The tropical cyclones of the Indian Ocean south of the equator also first pursue a westerly course, which gradually changes to south-west, and often on arriving about lat. 30° recurve to the south-east. Many of the cyclones of India have their origin to westwards of the Nicobar Islands, pursue a course to north-westward, and die out in the valley of the Ganges ; and, similarly, a considerable number of the cyclones of the West Indies pursue a westerly course through the Gulf of Mexico, and several die out in the States.
The most dreadful attendant on tropical cyclones is the storm-wave, caused by the in-blowing winds and the low pressure of the centre of the storm. When this wave is unusually high and is hurled forward on a low-lying coast at high water it becomes one of the most destructive agents known. The Bakarganj cyclone of October 31, 1876, was accompanied by a wave which flooded the low grounds to the east of the delta of the Ganges to heights varying from 10 to 45 feet, by which more than 100,000 human beings perished.
Tracks of Cyclones of North America, Atlantic, arid Europe. - In the Physical Atlas of the Atlantic Ocean, issued under the direction of Dr Neumayer of the Deutsche Seewarte, plate 28 shows by shadings the mean positions of the centres of cyclones and by lines their mean tracks. The following are the regions where the lowest barometer of storms has been most frequently found : - the region to west-south-west of the lakes of the United States ; the Gulf of St Lawrence ; mid-Atlantic about lat. 35° long. 52°; to the south-west of Greenland; to the south-west of Iceland, which is by far the most important of the whole ; to the south-west of the Lofoten Isles ; the region embracing Denmark, the south of Scandinavia, and Finland ; and, as secondary centres of frequency, the south of the British Islands, Corsica and part of Italy adjoining, and the northeast of the Adriatic. The great importance of these centres, where the lowest barometers are most frequently found, consists in the indication they give of the precise regions either where many storms originate or where they are either retarded or arrested in their course. As regards the origin of storms, the centre west of the Mississippi is the region where most of the United States storms originate, the centre in the Gulf of St Lawrence is where many of the great Atlantic storms have their origin, and the centres in mid-Atlantic and to the south-west of Iceland are the regions where the storms of north-western Europe chiefly originate. The centres on the south-west of Greenland, the Lofoten Isles, Denmark, and the south of the British Islands, all appear to suggest that storms are retarded in their onward courses on coming up against large masses of land, - which may, in part at least, be occasioned by the heavy rainfalls that mark these parts of their courses.
Of all storm tracks the most frequently taken is that by the storms of the United States, which pursue an easterly course through the lakes to the Gulf of St Lawrence. A considerable number of storms follow a course from Nova Scotia to Davis Straits ; but the larger number take a north-easterly- course through the Atlantic towards Iceland and thence past the north of Norway. Among the less frequent but important tracks are these : - from near New Orleans along the east coast of the States towards Nova Scotia ; from mid-Atlantic to south of Ireland and thence through France to the north of the Mediterranean ; and from the Atlantic about lat. 42° long. 40° in a northeasterly course quite outside but at no great distance from the British Islands, and thence towards the North Cape. Of the tracks more immediately affecting British weather are one from Iceland in a south-easterly direction through the North Sea and Germany, and four tracks which start from near Scilly: - (1) to the south-east as already described; (2) eastward through the north of Germany ; (3) north-east to Christiania ; and (4) north through Ireland and the Hebrides. These are the storm tracks which chiefly give the United Kingdom its easterly and northerly winds.
The Inclination of Winds to the Isobars. - The vorticose motion of the winds in a cyclone towards and in upon the centre has been already pointed out. One of the more important practical problems of meteorology is the determination of the angle of inclination of the winds to the isobars in the different segments of the cyclone, not only from the application of the results of the inquiry to the theory of storms but also to practical navigation. The first real contribution to the subject, based on accurate measurements, was made by Clement Ley in 1873.1 From the observations made at fifteen places in north-west Europe examined by him he showed that the winds incline from districts of higher towards those of lower pressure at a mean angle of 20° 51'; that the inclination is much greater at inland than at well-exposed stations on the coast, the respective angles being 28° 53' and 12° 49' ; and that the greatest inclinations are with S.E. winds. Then follow S.W., N.E., and N.W. winds, the last showing the least inclination. Whipple has recently compared the winds at Kew with the barometric gradients for the five years ending 1879, with the result that the greatest inclination is 63° with S.E. winds, the least 35° with N.E. winds, and the mean for all winds 52°.
As regards the open sea, Captain Toynbee has shown, from a careful investigation of the great Atlantic storm of August 24, 1873, that the mean angle of inclination calculated from one hundred and eight observations was 29°, the mean at the three selected epochs examined varying from 25° to 31°.
Barometric Gradient and Velocity of the Wind. - In inquiring into the relation of the velocity of the wind to the barometric gradient, it is necessary to have some definite information as to the increase of the velocity with height above the ground. Stevenson recently made observations on this point on winds varying from 2 to 44 miles an hour from the surface up to a height of 50 feet, from which he has drawn the following conclusions : - (1) the spaces passed over in the same time by the wind increase with height above the ground ; (2) the curves traced out by these variations of velocity from 15 to 50 feet high coincide most nearly with parabolas (fig. 20) having their vertices in a horizontal line 72 feet below the surface; (3) between 15 feet and the ground there is great disturbance of the currents, so that the symmetry of the curves is destroyed ; (4) the parameters of these parabolas increase directly in the ratio of the squares of the velocities of the different gales. If x be the velocity of the wind at height H above the ground, the parameter of the corresponding parabola is x2/(H + 72); and as x varies the parameter will vary as x2 or as the square of the velocity of the gale. It follows that, to render wind observations comparable, it is necessary that anemometers be placed at one uniform height above the ground, and that standard height not lower than 15 feet above the, surface. It is very desirable that the inquiry were prosecuted up to a height of 100 feet ; and it is of the utmost importance that the variation in the diurnal velocity be at the same time determined at different heights from 15 feet upwards.
Stevenson also made wind observations on the Calton Hill, Arthur's Seat, and the Pentland Hills, in the vicinity of Edinburgh, up to a height of 1600 feet above sea-level. It is from observations made at stations on knolls and peaks at different heights above the sox, and at different heights above the surfaces of their summits, that the problem of the variation of the wind's velocity at different heights with the same barometric gradient can be ascertained. In carrying the inquiry to considerable heights, the results cease to be comparable with those obtained at lower levels, unless in those cases where neighbouring heights are available for data from which the barometric gradient at the observed height can be calculated. The results of observations as to the velocity of atmospheric currents at very great elevations in the atmosphere deduced from the apparent movements of the higher clouds are altogether incomparable with the winds near the surface of the earth, for these among other reasons : - the heights of the clouds can be at best but imperfectly ascertained; the motion of the clouds, particularly the higher clouds, may be only apparent, it being sometimes difficult to distinguish between the formation and dissolution of clouds and their motion ; and above all, since the higher clouds are usually the accompaniments of the greater weather changes, their movements are the result of barometric gradients towards a knowledge of which we are absolutely powerless to take a single step.
As regards surface winds, Clement Ley in 1881, and Whipple more recently and with greater fulness, have calculated the mean wind velocities for twelve gradients, - the gradients being derived from the daily weather charts of the Meteorological Office for the five years 1875 to 1879 at 8 A.M., and the corresponding wind data being obtained from the hourly readings of the Kew anemograph. The barometric gradient is for 15 nautical miles, and the following are the velocities for the twelve gradients on the mean of the year: - The influence of season is very strongly marked. The velocities for the same gradients in order are - October to December, 12.5 miles ; July to September, 12.6 miles ; January to March, 14•S miles ; and April to June, 17-2 miles. From those observations of Whipple it follows that during the six months when the temperature is falling the velocity for the same gradients is least, while the velocity is greatest during the six months when the temperature is rising, and absolutely greatest during the three months ending June, when the greater part of the annual increase of temperature occurs. It is evident that the observed increase in the velocity of the wind for the same gradients is to be referred to the same cause that brings about the diurnal increase in the wind's velocity, viz., the wind blowing over a warmer surface than itself.
Whipple has also sorted the winds according to the eight points of the compass, with results of the greatest interest. If N.W., N., N.E., and E. winds be grouped together as polar, and S.E., S., S.W., and W. winds as equatorial winds, the mean hourly velocity of the polar winds, for the same gradients, is 1.1 miles in excess of the equatorial winds. Now, since polar winds pass into lower latitudes, the surface of the earth over which they blow is warmer, whereas the surface is colder than the equatorial winds which blow over it. It follows that the increased velocity of polar winds is referable to the same conditions which result in the diurnal increase in the wind's velocity and the greater velocity for the same gradients of winds when the annual temperature is rising, since in all these cases the winds blow over a surface of a higher temperature than their own.
It is evident from these considerations that for the development of the law of the relation of the wind's velocity to the barometric gradient with an exactness sufficient to warrant us in expressing that relation in a general mathematical formula much yet remains to be done. In truth, as regards the various formulte submitted by Ferrel, Mohn, Hann, Everett, and others, we have no choice but to allow the justness of Strachan's criticism (Modern Meteorology, p. 98) that the theoretical values furnished by the formula do not accord with the actual values, and that therefore a satisfactory formula is yet to be found. Ere such a formula need be looked for, the conditions must be fulfilled for the preliminary work of supplying the observational data required. The "Challenger" observations prove that, with gradients substantially the same, the velocity of the wind is greater on the open sea than near land ; and we have seen that the velocity varies with the hour of the day, and generally is increased as the temperature of the surface rises above that of the air blowing over it, and diminished as the temperature of the surface falls below that of the air. It is evident that observations on the open sea will afford data for the simplest solution of the problem ; but on land the diurnal, seasonal, and non-periodic changes of temperature greatly complicate the problem, and render necessary for its solution observations specially designed for the purpose. It is not easy to see how these can be obtained but by carrying out the plan proposed in 1875 by Stevenson of establishing strings of well-equipped meteorological stations planted sufficiently close that the barometric gradients may be determined within the limits of accuracy required. Observations made twelve times daily for a year, at stations so arranged, would supply the observational data for the solution of this fundamental problem in meteorology. Till some such proposal be carried out, the problem remains unsolved, for barometric gradients based on the widely separated existing stations are too uncertain and rough and the wind observations are wanting in that comparability which alone can satisfy the inquiry.
Weather and Weather Maps. - Weather is the state of the air at any time as respects heat, moisture, wind, rain, cloud, and electricity; and a change of weather implies a change in one or more of these conditions. Of these changes the most important as regards human interests are those which refer to temperature, wind, and rain; and, as these are intimately bound up with the distribution of atmospheric pressure, the latter truly furnishes the key to weather changes.
These relations are well shown by the International Monthly Weather Maps issued by the United States Signal Service. Of these that for December 1878 is a striking example. This month was characterized over the globe by unusually abnormal weather. A line drawn from Texas to Newfoundland, across the Atlantic, the north of France, and Germany, thence round to south-east, through the Black Sea, the Caucasus, India, the East India Islands, and Australia to the South Island of New Zealand, passes through a broad and extended region where pressure was throughout considerably below the mean of December, and this low pressure was still further deepened in various regions along the line. Another line passing from Australia, through the Philippine Islands, Japan, Manchuria, Behring's Strait, and Alaska, also marks out an extensive region where pressure was uninterruptedly below the mean.
On the other hand, pressure was above the average, and generally largely so, over the United States to west of longitude 90°, over Greenland, Iceland, the Fames, Shetland, and a large portion of the Old Continent bounded by a line drawn from Lapland round by Lake Balkhash, Canton, Peking, to the upper reaches of the Lena. Another area of high pressure extended from Syria, through Egypt and East Africa, to the Cape ; and part of a third area of high pressure appeared in the North Island of New Zealand. As regards North America, the greatest excess of pressure, 0.196 inch above the mean, occurred in the Columbia Valley, from which it gradually fell on proceeding eastward to a defect from the average of 0.146 inch near Lake Champlain and to northward, rising again to near the mean on the north of Nova Scotia. To the north and north-east exceedingly high pressures for these regions and the season prevailed, being 0-635 inch above the mean in Iceland, 0.500 in the south of Greenland, and at the three stations in West Greenland, proceeding northward, 0.445, 0.402, and 0346 inch.
West Greenland being thus on the west side of the region of high pressure which occupied the northern part of the Atlantic, and on the north-east side of the area of low pressure in the States and Canada, strong south winds set in over that coast, and the temperature at the four Greenland stations, proceeding from south to north, rose to 1°.1, 12•1, and 14'4 above the means. As the centre of lowest pressure was iu the valley of the St Lawrence about Montreal, strong northerly and westerly winds predominated to westward and southward, where consequently temperature was below the average, the deficiency at Chicago and St Louis being 9°'5; and, winds being easterly and northerly in California, the temperature there was also under the mean. On the other hand, in the New England States, the greater part of the Dominion of Canada, and West Greenland temperature was above the average. Pressure was much higher at St Michael's, Alaska, than to south-westward at St Paul's, Behring's Strait, and hence, while temperature at St Paul's was 2°.9 below the normal, it was 12'0 above it at St Michael's, where strongly southerly winds ruled. With these strong contrasts of pressure, America presented contrasts at least as striking in the distribution of the temperature. Along the south of Lake Michigan the November temperature was 13'7 above the normal, whilst the December temperature was 90•5 below it, the difference there between the two consecutive months being thus 23°'2.
As regards Europe, Iceland was on the east side of the patch of high pressure which overspread the north of the Atlantic, and hence northerly winds prevailed there and temperature fell 7°-2 below the mean, presenting thus a marked contrast to the high temperature of West Greenland at the time. In Europe, the area of lowest pressure occupied the southern shores of the North Sea, extending thence, though in a less pronounced form, to south-eastward. Hence over the whole of western Europe winds were N.E., N., and in the south-west of Europe W.; and hence everywhere from the North Cape to the north of Italy temperature was below the normal, in some places greatly so, the deficiency being 10'4 in the south of Norway and 12•2 in the south of Scotland. On the other hand, on the east side of this area of low pressure winds were southerly and temperature consequently high. In some localities in Russia the excess above the mean was 150-0, and oter a large proportion of European Russia the excess was not less than 9'0. This region of high temperature extended eastward into Siberia as far as the Irtish, being coterminous with the western half of the anticyclonic region of high pressure which overspread central Siberia. But over the eastern portion of the anticyclone northerly winds prevailed, with the inevitable accompaniment of low temperatures over the whole of Eastern Asia, the deficiency at Nertchinsk on the upper Amur being 6°.8. Here again, just as in America, Greenland, and Iceland, places with atmospheric pressure equally high presented the strongest contrasts of temperature. Thus at Bogoslovsk, on the Ural Mountains, pressure was 0.211 inch and at Nertcbinsk 0.154 inch above the normals, but Bogoslovsk on the west side of the high pressure area had a temperature 150.0 above, whilst at Nertchinsk it was 6°-8 below the average.
At this time of the year the mean pressure falls to the minimum in Australia, but during December 1878 the usually low pressure was still further diminished. Pressure at this season also falls to the annual minimum in the North Pacific and North Atlantic, and it has been seen that the low pressure of these regions was likewise still further diminished. But in the case of the Atlantic it was attended with a most important difference. The centre of lowest pressure, usually located to the south-west of Iceland, was removed some hundreds of miles to the south-east, and an unwonted development of extraordinarily high pressure appeared to the northward, overspreading the extensive region of Baffin's Bay, Greenland, Iceland, Faroes, and Shetland. It was to this region of high pressure, particularly in its relations to the low-pressure region to the south-east of it, that the extreme severity of the weather in the British Islands at the time was due. Now this high-pressure region was intimately connected with, and doubtless occasioned directly by, upper atmospheric currents from the widely extended region of low pressure to southward, with its large centres of still lower pressure in the North Sea, mid-Atlantic, and United States, where pressures were respectively 0.307, 0-322, and 0.146 inch under the normals. Thus, with the single exception of the high-pressure area about Greenland, the meteorological peculiarities which render December 1878 so memorable over nearly the whole globe arose out of a distribution of the earth's atmosphere essentially the same that obtains at that time of the year, but the usual irregularities in the distribution of the pressure appeared in more pronounced characters.
Taking the all-important bearings of these areas of high and low pressure on weather and climate into consideration, along with the abnormal concentration of aqueous vapour over extensive regions which they imply, it is evident that, when the meteorologist will be in a position to forecast, on scientific grounds, the weather of the coining season for the British Islands, it is to the Atlantic he will require to look for the data on which the forecast is based.
These questions, which the International Weather Maps of the United States enable us to discuss, are of the first importance in meteorology, whether we consider the amplitude of the atmospheric changes they disclose (these being often so vast as to embrace four continents at one time, besides being profoundly interesting from their direct bearings on the food supplies and commercial intercourse of nations) or regard the larger problems they present, with hints towards their solution, which underlie physical geography, climatology, and other branches of atmospheric physics. The discussion presents the great atmospheric changes as influenced by oceans and continents, including the subordinate but important parts played by mountain ranges, extensive plateaus, and physically well-defined river basins in determining the development, course, and termination of these changes.
Weather Forecasts and Storm Warnings. - It is in tropical and subtropical countries that an isolated observer may, with a close approximation to certainty, predict the approach of gales and hurricanes. In these regions atmospheric pressure and the other meteorological conditions are so constant from day to day that any deviation, even a slight one, from the average of the hour and season in respect of pressure, the direction and strength of the wind, and the direction and amount of cloud, implies the presence of a storm at no great distance. Dr Meldrum has practically worked out this problem at Mauritius with great success. At the Royal Alfred Observatory there the mean pressure at sea-level in January at 9 A.M. is 29.966 inches, from which it falls to 29.904 inches at 4 P.M., then rises to 29.980 inches at 10 r.ai., and again falls to 29.927 at 4 A..M. The mean direction of the wind and the diurnal variation, both as regards direction and force, have been stated (p. 125). Suppose then that the barometer is observed to fall after 9 A.M. more rapidly than is due to the usual daily barometric tide, that in the afternoon it does not indicate the second maximum or that it continues to fall instead of rising, - or suppose, in short, any deviation from the mean daily march, - then it is certain that there is somewhere an atmospherical disturbance near enough to Mauritius to influence the pressure. The direction in which the disturbance is from Mauritius is readily known from the wind, and the distance of the storm closely approximated to by noting the rate and amount of the fall of the barometer, in connexion with the changes of the wind and the clouds, - the rate and progressive motion of the storm being known chiefly from the veerings of the wind. For a good many years past notifications have been sent to the daily newspapers when observations show that a storm is not far from the island, stating its position and probable course from day to day. The scheme of storm warnings at Mauritius has been entirely successful, and the result is of great value, since it shows what may be done at an isolated station in the ocean, or what may be done in ships at sea. In this connexion it is not possible to overestimate the importance to seamen of a knowledge of the hourly variations of the barometer and its mean monthly heights over the ocean tracks of commerce.
In passing from Mauritius to the British Islands we pass from a region where the forecasting of storms and weather is simplest and easiest to the region where it is most complex and difficult, particularly for the western districts of these islands. The great difficulty lies in the fact that the British Islands are immediately bounded by the Atlantic to westwards ; and, since practically every storm and nearly all weather changes come from that direction, no telegraphic communication of their approach can be received. The Meteorological Office in London has therefore no choice but to base the forecasts on such of the observations telegraphed to the office as experience has shown to be the precursors of storms and other weather changes. The more important of these observations are the falling and rising of the barometer taken in connexion with changes in the direction and force of the wind. Since on the north side of the track of the centre of the storm winds are northerly and easterly and temperature low, and on the south side winds are southerly and westerly and temperature high, one of the most important points to be ascertained is the probable path the centre of the coming storm will take. Though a good deal remains to be accomplished in the development of this phase of storms, yet much has recently been done in this direction by close examination of the changes of pressure in the region of the anticyclone contiguous to the advancing storm and by the changing positions of the rain area near the centre of the cyclone.
As regards Europe, the facility of forecasting storms increases as distance from the west coasts is increased. Thus to the middle and eastern districts of the British Islands, were a day and night watch established in the west, forecasts of almost every storm could be issued, the exceptions being those small cyclones or satellite cyclones, as they are called, originating within the British Islands themselves, which are frequently characterized at once by-their severity and by the rapidity of their onward course. In the United States, the system of weather forecasting is perhaps the best in temperate regions, - a result due to the admirable system organized and developed under the direction of the late General Myer, and adequately subsidized by the Government, but above all to the facilities to detect and track the storms in the region where nearly all of them have their origin, to west of the Mississippi, before they advance upon the more thickly peopled States.
Meteorology sustained a heavy loss by the death in 1877 of Leverrier, who was not only the keenest-sighted of physicists but also the prince of organizers of systems of meteorological observation. His last great service to the science was the establishment of a system of observation, by which the propagation of rain, hail, and other weather phenomena could be followed and recorded from commune to commune over France. This scheme for the investigation of the vitally important bearing on the meteorology of a country of a comprehensive observation of its rainfall, hail, and thunderstorms, through numerous observers possessing sound local information, is not only eminently just in science, but is calculated to be attended with the greatest benefits to agricultural and other public interests. The practical advantages of the scheme, it need scarcely be added, can only be reaped after a very large expenditure of labour and money in organizing a comprehensive parochial scheme of observation, systematically and persistently carried through and discussed.
Further details regarding meteorological phenomena will be found in the articles ATMOSPHERE, BAROMETER, CLIMATE, HYGROMETRY OZONE, RAINGAITGE, SEA, and THERMOMETER. (A. B.)