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Q: Why do droughts occur?
A: A drought is defined by the National Weather Service as a deficiency of moisture that results in adverse impacts on people, animals or vegetation over a sizable area, says Steven Quiring of Texas A&M University. “Droughts are a creeping phenomenon,” says Quiring, a geographer who is establishing a database of soil moisture information for the U.S. Great Plains. “Droughts usually develop and spread slowly over time. This makes it difficult to identify the start of a drought and to measure and monitor drought conditions. Droughts are a direct response to a shortage of rainfall and/or an increase in evaporation and transpiration. The lack of rain can be attributed to many factors, but two main underlying reasons prevail. For rainfall to occur, there must be moisture in the atmosphere and an upward direction of that moist air. Any time one or both of these are absent, rainfall can become scarce. This combination can be aggravated by persistent high pressure areas, delivery of only dry, continental air masses, or proximity to high pressure ridges.”
Q: What factors make droughts worse?
A: Many aspects of urban and suburban development affect drought, Quiring says. “Deforestation and over-farming can contribute to drought severity and occurrence because these activities reduce soil moisture,” he explains. “Hydrological drought can occur if a river or water source is depleted upstream through irrigation or an increase in demand. Many times drought conditions can be exacerbated by a feedback effect -- decreases in rainfall lead to decreases in soil moisture or plant growth, which reduces the amount of moisture available to the atmosphere. Without this moisture, it can steadily become more difficult for rainfall to occur.”
Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University. Visit http://tamunews.tamu.edu. Follow us on Twitter at http://twitter.com/tamu.
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