by David Herring · September 25, 2000

After the “dust bowl” years of the 1930s—in which much of the Great Plains region of North America suffered extreme drought conditions for years with devastating economic consequences—overall climate conditions have grown wetter with each passing decade. Station rain gauge records around the world reveal that over the last century annual precipitation has increased by about 2.4 mm per decade, on average, with much of the increase concentrated in North America (Dai et al. 1997).
 

But despite the general increase in rainfall, in almost any given year meteorologists can point to a region on the map that experienced drought. Precipitation patterns can vary widely on regional scales, from one season to the next, and from year to year. It is hard to predict accurately where and how much rain will fall next week. It is harder still to forecast next year's rainfall patterns.

Consider, from 1995 to 1996, the northwestern and northeastern regions of the U.S. received higher than normal rainfall amounts, while precipitation in Alaska, eastern Canada, and the central and southwestern U.S. was substantially below normal (Bell and Halpert 1997). Then, from January through the summer of 1998 there was excessive rainfall in California and across much of the central and southern United States, while the Gulf Coast states and Mexico experienced severe drought (Bell et al. 1998). In the summer of 2000, drought hit hard in southeastern states like Georgia and Alabama while much of Texas has gone more than 67 consecutive days without rain. An increasing number of scientists say we are in for more extreme "wet" and "dry" events, and we can expect them more often as average global temperatures rise (Dai et al. 1998).

Poring over data from satellite remote sensors, ground-based rain gauges, shipborne measurements, and even historical samples taken from sediment strata in dried rivers and lakebeds, researchers are painstakingly piecing together a picture of dry spells in North America. Where and how often do droughts typically occur, and with what intensity? And, perhaps most important, what causes them? The goal is to use these new data to construct computer models that will help scientists predict the onset of a drought weeks, months, or even a year ahead of its arrival.

 Physical Processes that Cause Drought