A prolonged period of unusually low precipitation is called a drought.
Drought develops when water availability remains below normal for long enough to affect ecosystems, agriculture, communities, or economies. It can result from persistent lack of rain or snow, unusually high temperatures, strong evaporation, reduced river flow, or a combination of these factors.
Scientists distinguish several types. Meteorological drought concerns below-average precipitation; agricultural drought affects soil moisture and crops; hydrological drought involves reduced rivers, reservoirs, and groundwater; and socioeconomic drought occurs when water shortages disrupt people and industries.
Drought does not have one universal duration or threshold. A dry spell that is severe in a normally wet region may be ordinary in a desert. It is also different from aridity, which describes a region’s long-term dry climate rather than a temporary shortage of water.