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

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Droughts occur in nearly all types of climate. Of all the weather-related phenomena that can cause severe economic impacts in the United States, droughts come in second only to hurricanes, according to the National Climatic Data Center. But unlike hurricanes, which are easily identified and straightforward to classify in terms of wind speeds, droughts are much tougher to define.

Definition of drought

Most people think of a drought as a period of unusually dry weather that persists long enough to cause problems such as crop damage and water supply shortages. But because dry conditions develop for different reasons, there is more than one definition of drought.

"Drought is caused by not only lack of precipitation and high temperatures but by overuse and overpopulation," said David Miskus, a drought expert and meteorologist at the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center.

These definitions usually specify the beginning, end and degree of severity of drought by comparing the precipitation over a certain time period to a historical average. Scientists include both rain and snow in precipitation measurements, because some U.S. regions, such as the mountainous West, rely on winter snow for much of their yearly water.

Here are descriptions of the four main categories of drought:

Meteorological drought is specific to different regions, depending on the amount of yearly precipitation that's average for that area. For example, the southwest portion of the United States averages less than 3 inches (7.6 centimeters) of precipitation per year, while the Northwest gets more than 150 inches (381 cm) per year, according to the U.S. Department of Interior. A decrease in precipitation compared to the historical average for that area would qualify as a meteorological drought.  

Agricultural drought accounts for the water needs of crops during different growing stages. For instance, not enough moisture at planting time may hinder germination, leading to low plant populations and a reduction in yield.

Hydrological drought refers to persistently low water volumes in streams, rivers and reservoirs. Human activities, such as drawdown of reservoirs, can worsen hydrological droughts. Hydrological drought is often linked with meteorological droughts.

Socioeconomic drought occurs when the demand for water exceeds the supply. Examples of this kind of drought include too much irrigation or when low river flow forces hydroelectric power plant operators to reduce energy production.