Is it possible for droughts to become more common even if annual rainfall amounts increase? Overall annual precipitation in the lower 48 states has been increasing since the early 20th century, and since the 1970s it has been increasing in the eastern United States by about one-inch per decade. Over the last forty years, this region has experienced another trend: an increase in the frequency of thirty-day periods during warm months when there is no rain. These dry spells now occur about twice as often. While these trends may seem contradictory, it appears that the lack of rainfall during these dry periods is more than compensated for by an increase in the frequency of heavy rains. Heavy rainfall events are now 14 percent more frequent than they were three decades ago, while extreme rainfall events are seven percent more frequent. The frequency of moderate rainfall events has decreased slightly.
Seasons: Spring, Summer, Fall
Sources: Groisman, PY and Knight RW. “Prolonged Dry Episodes over the Conterminous United States: New Tendencies Emerging during the Last 40 Years.” Journal of Climate 21 (2008): 1850-1862 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 and Trenberth, K et al. “The Changing Character of Precipitation.” Bulletin of the American Meteorological Society, September 2003: 1205-1217 and Easterling, D et al. “Observed climate variability and change of relevance to the biosphere.” Journal of Geophysical Research 105 (2000): 101-120.
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