At its most basic, drought is a lack of precipitation over an extended period of time. However, according to the National Drought Mitigation Center
, what constitutes drought varies from region to region, depending on the local climate. Some areas naturally receive more rainfall than others, and other climatic factors such as temperature, humidity and wind speed play a role as well. Prolonged droughts
are characterized by large-scale anomalies in atmospheric circulation patterns that persist for months, seasons or even longer. Meteorologists generally compare current rainfall patterns to historic averages to determine when a region is experiencing a drought and how severe it is.
Drought is also based on need. Drought occurs when water supply does not meet water demands by people, animals and plant life. In addition, different groups of people view drought differently. Growers, for example, generally consider drought in terms of crop impacts, as soil moisture may indeed diminish quickly during periods of low rainfall.
The source of water used may also determine the severity of drought. For example, groundwater supplies may not be impacted as quickly as surface water supplies are in a drought. On the other hand, groundwater may take longer to recover from a severe drought.
Drought is also sometimes man-made. For centuries, we have altered our surroundings to protect ourselves from danger—storms, cold and even drought. By building dams and reservoirs, we can control flooding and provide a more reliable source of water for surrounding communities. However, sometimes increased use upstream can cause a deficit in supply for downstream stream neighbors.