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July/August 2008

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Retrospect: July 4, 1956

Like all meteorological phenomena, rainfall can be categorized in terms of extreme events. The most rainfall in a single year, for example, occurred in Cherrapunji, India, when 1,042 inches fell from August 1860-July 1861. To put this in perspective, most locations in the conterminous United States average fewer than 70 inches of precipitation per year.

Rainfall records on shorter time scales—say, an hour or less—tend to be associated not with large, synoptic scale systems,  such as tropical and midlatitude cyclones, but rather with smaller, mesoscale events, such as the convective thunderstorms that pop up across the United Sates during summertime. Such was the case on July 4, 1956, in Unionville, Maryland. On this day, residents of nearby northern Virginia and north-central Maryland experienced some of the heaviest rainstorms in years. One of these storms resulted in what the U.S. Weather Bureau’s Weekly Weather and Crop Bulletin referred to as a “freak cloudburst” over Unionville, about 60 miles north of Washington, D.C., producing a storm with total rainfall of 3.60 inches from 2:50 p.m.-11:50 p.m. The bulk of this rain—2.84 inches—fell during a 50-minute period from 2:50 p.m.-3:40 p.m.

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