What is a Drought? Definition of Droughts

Of all the weather-related phenomena that can cause severe economic and environmental damage in the United States, droughts have historically had the greatest impact on the largest number of people, according to the National Climatic Data Center. But unlike hurricanes or tornadoes, which are easily identified and straightforward to classify in terms of wind speeds, droughts are much tougher to define.

Extreme weather such as heat waves, heavy downpours and droughts are expected to accompanying climate change. Recent research indicates this has begun happening.
Extreme weather such as heat waves, heavy downpours and droughts are expected to accompanying climate change. Recent research indicates this has begun happening.
Credit: Dreamstime

Definition of drought

In simple terms, a drought is a period of unusually dry weather that persists long enough to cause environmental or economic problems, such as crop damage and water supply shortages. But because dry conditions develop gradually and impact different regions differently, there's no agreed upon way to pinpoint when a drought begins or ends, or to objectively assess its severity.

Of the many schemes for classifying droughts, the most widely used is the Palmer Drought Severity Index (PDSI), which combines temperature, precipitation, evaporation, transpiration, soil runoff and soil recharge data for a given region to produce a single negative number representing conditions there. This index serves as an estimate of soil moisture deficiency, which roughly correlates with a drought's severity, and thus, its impacts.

Drought map - July 3, 2012
Credit: Rich Tinker/NOAA/NWS/NCEP/CPC

Drought maps

The U.S. Drought Monitor, an organization run by government and academic partners that maintains a nationwide drought map, uses the PDSI to categorize dry weather into five levels of severity:

  • abnormally dry (category D0, corresponding to a PDSI between -1.0 and .1.9)
  • moderate drought (D1, PDSI between -2.0 and -2.9)
  • severe drought (D2, PDSI between -3.0 and -3.9)
  • extreme drought (D3, PDSI between -4.0 and -4.9)
  • exceptional drought (D4, PDSI between -5.0 and -5.9)

The effects range from slow crop and pasture growth to widespread crop failure and water emergencies. Additionally, the Drought Monitor defines droughts as either short-term, if they've lasted less than six months, and long-term for prolonged events.

Causes of drought

Abnormally low rainfall is, of course, the primary cause of drought. But one can't say in general how little rainfall it takes for a region's Palmer index to sink into drought territory, because the index takes regional averages into account. For example, 20 inches of rainfall in a year is normal in West Texas, and would correspond to a Palmer index around 0; but 20 inches would be less than half the yearly average in Virginia, and would probably correspond to an index lower than -5.0, signifying an exceptional drought in the state.

This regional specificity of the categorizations makes sense in terms of land usage: Wetter regions tend to be filled more densely with people, wildlife and crops, and so more rain is required to maintain normal conditions.

A dry future?

However, as the human population increases in arid regions as well as wet ones, so will the demand for water and — with water supplies dropping at a faster rate — so will the likelihood of drought. In fact, population booms can trigger droughts almost by themselves.

A severe drought that gripped the Southeast from 2005 to 2007 was largely attributed by climate scientists to a 50 percent rise in the region's population during the preceding 15 years, which placed unprecedented demands on the water supply.

Aside from the human population explosion, global warming is also expected to fuel increased frequency and severity of droughts in many parts of the globe in future. According to projections by the Intergovernmental Panel on Climate Change, droughts will especially increase in subtropical areas, such as the U.S. Southwest, Australia and parts of Africa and Europe, as Earth's warming causes more evaporation and shifts weather patterns, pushing the paths of storms that bring thirst-quenching rains farther north.

Dust storm approaching Stratford, Texas.
Credit: NOAA Photo Library, Historic NWS collection

History of U.S. droughts

Droughts have been cited as a scourge of humankind since biblical times, but the nation's most devastating drought on record occurred in the 1930s during the so-called "Dust Bowl" years. According to the National Climatic Data Center, the drought affected almost the entire Plains and covered more than 60 percent of the country at its peak in July 1934. It caused the migration of millions of people from the Plains to other parts of the country, especially the West Coast.

More recent droughts, such as those of the 1950s, 1988 and 2000, have also had serious economic and societal impacts. Between 1980 and today, 16 drought events have cost a combined $210 billion in the United States, and thousands of people have died due to the effects of drought.

In June 2012, 55.79 percent of the land in the lower 48 U.S. states were in drought, the highest figure in the 12-year history of the U.S. Drought Monitor.

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