What About Drought? - The Causes Of Drought
air winds blocking clouds
Nearly all droughts are caused by changes in the patterns and movement of winds, which carry rain-giving clouds. Rain, of course, comes from clouds. Generally, clouds form when wind forces air to move upward. When wind forces air to move downward, clouds disappear. And so does the rain.
The west-to-east flow of winds at mid-latitudes such as North America brings what we think of as normal weather. In spring, as polar air moves north, winds flow in a more north-south direction. Occasionally, this change in direction makes wind patterns unstable. The unstable winds begin to circle in on themselves, forming high-pressure, circular cells. Sometimes, these high-pressure cells remain stationary over a region for a long time, forming a blocking high. A blocking high causes a sinking, downward flow of air that prevents cloud formation. A blocking high is also an obstacle to cool, moisture-laden air, which flows around it The land beneath the blocking high thus gets no clouds and no rain. If the blocking high stays put for too long, a drought may result.
In North America, winds flow from west to east on the jet stream, a large river of air several miles above the surface of Earth. In normal years, warm, moist air from the Caribbean moves north and joins the jet stream. When the westerly winds are blowing with their usual force, they absorb the southern winds and carry them eastward. On this journey, the warm air cools, clouds form, and rain falls as the air mass moves across the continent. During some years, conditions in the ocean and atmosphere change, blocking highs form, and drought results.