Drought

Drought -- the temporary absence of enough water to support the usual local ecosystem -- is an ancient, timeless weather event that can happen because of unusual weather, human activity, or a mixture of the two. Any prolonged lack of rain can result in a local or widespread drought, and a lessening of usual precipitation can make conditions ripe for a drought should any further change in water supply occur. There are also areas of the world that are subject to periodi droughts because of the local climate's nature.

Some droughts, such as the Dust Bowl of the United States, have become famous enough to be named. Others, such as the droughts mentioned in the Bible -- "who can stay the bottles of heaven, When the dust groweth into hardness, and the clods cleave fast together" -- are known only by indirect reference, but show that humanity has experienced droughts since antiquity and far beyond. There are several kinds of drought, and each one has interest from a meteorological point of view as well as being important for those forced to undergo it.

Meteorological drought

The most obvious form of drought is meteorological drought, when the rainfall over an area is much less than usual for long enough to start affecting the local plant life and agriculture. Although it is far less violent than many of the damaging weather patterns -- such as tornadoes, hurricanes, or derechos -- drought can have effects that are no less devastating in the long run. If a drought is large enough and persistent enough, it can trigger conflicts by forcing people to migrate into areas inhabited by other people, kill livestock, and cause starvation among those who cannot escape it.

An especially persistent blocking high can cause a fairly small, temporary drought. A blocking high is a high pressure air mass that remains steadily in one place, preventing other air masses from moving into the area and thus also preventing rainfall. These highs sometimes arrive in the summer, and are associated with hot, sunny, dry weather which, together with the lack of rain, results in steadily more parched conditions. Ironically, the edges of the blocking high may witness higher than usual rainfall, as other air masses 'deflect' off the high. This was seen on a small scale several years ago when Wisconsin crop fields were parched for weeks during the summer, while in Minnesota, the Mississippi River flooded due to the constant, powerful rains.

Larger shifts in the Earth's weather engine can cause more massive, persistent droughts. The jet streams which traverse the world are responsible for moving tropical moisture north into the temperate regions. If there is a period when the jet stream weakens or follows a different path than it normally does, then this moisture can be diverted away from where it is usually found and a huge, long-lasting drought results. An example of this is the Dust Bowl of the U.S. in the 1930s, when the jet stream failed to bring moisture to the central U.S. and the result was economic and demographic disaster, complete with dust storms known as "black blizzards."

Hydrological drought

Hydrological drought is more insidious and possibly more persistent than meteorological drought because it is caused by depletion of ground water. A meteorological drought ends fairly rapidly when rainfall returns to normal, while a hydrological drought is far more complex, long-lasting, and may cause irreparable change to local conditions.

An area may experience hydrological drought when an area from which water flows into its ground water layer ceases to provide that water, for example. If a river changes course, human activity drains the water table faster than it can be replenished, or the weather patterns change somewhere else that a region depends on, then the ground water may never return to its previous levels.

Hydrological drought is also more likely to be directly caused by human activity. There is no human action that can directly alter the flow of a jet stream, but anyone with a pipe and a pump can change the flow of a watercourse. The Aral Sea is currently being drained by several former Soviet republics, leading to hydrological drought in areas of Kazakhstan.

The effects of drought

The most obvious effects of drought include the drying and withering of plant life. When this is enough to damage crops, then the drought has become strong enough to be an 'agricultural drought.' Drought can also lead to the loss of topsoil, as the soil dries out, and the wind literally blows it away in dust storms. Even when rain returns, the area's ability to support agriculture and plant life may have been seriously and permanently altered. For this reason, modern nations take as many steps as they can to lessen the effects of drought, hoping to prevent permanent damage to their precious agricultural land.