Droughts

A drought is defined as "a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area." - Glossary of Meteorology (1959).

In easier to understand terms, a drought is a period of unusually persistant dry weather that persists long enough to cause serious problems such as crop damage and/or water supply shortages. The severity of the drought depends upon the degree of moisture deficiency, the duration, and the size of the affected area.

Based on the disciplinary perspectives, drought can be defined as follow.

Meteorological Drought

Meteorological drought is usually defined based on the degree of dryness (in comparison to some average) and the duration of the dry period. Definitions of meteorological drought must be considered as region specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable from region to region.

Agricultural Drought

Agricultural drought links various characteristics of meteorological (or hydrological) drought to agricultural impacts, focusing on precipitation shortages, soil water deficits, reduced ground water or reservoir levels needed for irrigation, and so forth. In other words, it refers to a situation where the amount of moisture in the soil no longer meets the needs of a particular crop.

Hydrological Drought

Hydrological drought usually occurs following periods of extended precipitation shortfalls that impact water supply (i.e., streamflow, reservoir and lake levels, ground water), potentially resulting in significant societal impacts. Because regions are interconnected by hydrologic systems, the impact of meteorological drought may extend well beyond the borders of the precipitation-deficient area.

Below is a graph that describes drought in connection to the above-mentioned Drought. The economic, social and environmental impacts are shown at the bottom. This graph also explains that drought can occur at any stage.

Source and find more about drought:
http://www.drought.unl.edu/whatis/what.htm