Many locations throughout the Pacific Northwest and the Rocky Mountain States were under severe drought conditions for much of the summer. Significant monetary losses were felt, with lost crops and the losses from some of the worst wildfires in more than a half century.
What constitutes a drought? The answer depends upon whom you ask. At least four types of drought can be defined. To the farmer, an agricultural drought represents an extended interval with a serious soil moisture deficiency during critical crop growth periods. A hydrologist would classify a hydrological drought as an extended interval containing abnormally low stream flow, lake levels and ground water reservoirs. Most meteorologists would consider a meteorological drought to occur when the accumulated precipitation is well below a prescribed amount that would depend upon the region or season. A fourth type would be socioeconomic drought, where the shortage of water affects humans, typically in terms of economic activities. Economic goods that may be affected may include water, agricultural food products and hydroelectric power.
Typically, the severity of a drought depends upon the lack of soil moisture, which is influenced by one or more of the following factors to include a lack of precipitation, low atmospheric humidity, high air temperature, strong winds, a lack of clouds and intense sunlight. In addition, the drought severity depends upon its duration and the size of the affected area.
The start of a drought usually is subtle in that few can tell when a spell of dry weather really constitutes the incipient phase of a drought. Similarly, the end of a drought is also difficult to assess, since one rain event does not necessarily "break a drought". The National Weather Service uses several indices to assess the severity of a drought. One of the most frequently used drought indices is the Palmer Drought Severity Index developed by Wayne Palmer in the 1960s. This Palmer Index incorporates temperature and rainfall information in a formula to determine abnormal dryness or wetness over prolonged time intervals, such as a month to years. The National Weather Service and U.S. Department of Agriculture jointly compute the Drought Index weekly for each of 344 climatological divisions across the United States. A map of the current Drought Index is available that shows those divisions experiencing drought with negative index values and varying shades of red, while those regions with excess precipitation have positive values and varying shades of green.
The most recent map (as of 6 October 2001) shows a broad region of moderate to extreme drought across much of the West, with the Pacific Northwest and the northern Rocky Mountain States continuing to experience extreme drought. East of the Rockies, several areas also were under moderate to severe drought conditions to include northern New England and northeastern Arkansas, while moderate drought was also reported in portions of the Carolinas. Near normal conditions prevailed over most of the rest of the nation, except for several regions across the central Gulf Coast, the Midwest and Dakotas where recent rains have produced unusually moist soil conditions. Some divisions in Kansas, Missouri, Nebraska and North Dakota are experiencing excessively moist conditions.
Palmer, W.C., 1988 (12 Jul): The Palmer Drought Index: When and how it was developed. Weekly Weather and Crop Bulletin, 75 (28), 5.
URL Address: datastreme/learn/w_sup.html
Prepared by Edward J. Hopkins, Ph.D.
© Copyright, 2001, The American Meteorological
Society.
