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Droughts in the United States
Drought in the United States is similar to that of other portions of the globe. Below normal precipitation leads to drought, which is caused by an above average persistence of high pressure over the drought area. Changes in the track of extratropical cyclones, which can occur during climate cycles such as the El Niño-Southern Oscillation, or ENSO, as well as the North Atlantic Oscillation, Pacific Decadal Oscillation, and Atlantic Multidecadal Oscillation, modulates which areas would be more prone to drought and when drought develops. Increased drought frequency is expected to be one of the effects of global warming. In dry areas, removing grass cover and going with a more natural vegetation for the area can reduce the impact of drought, since a significant amount of fresh water is used to keep lawns green. Droughts are periodic, alternating with floods over a series of years.
The worst droughts in the history of the United States occurred during the 1930s and 1950s, periods of time known as 'Dust Bowl' years in which droughts lead to significant economic damages and social changes. In particular, relief and health agencies became overburdened and many local community banks had to close.
==Metrics==
The U.S. Drought Monitor provides national database to track the duration and severity of droughts in the United States, hosted by the University of Nebraska-Lincoln with assistance from the United States Department of Agriculture and the National Oceanic and Atmospheric Administration. Their standardized measurements track droughts on a severity scale from "Abnormally Dry" (D0) to "Exceptional" (D4).〔(【引用サイトリンク】url=http://droughtmonitor.unl.edu/AboutUs/ClassificationScheme.aspx )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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