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.

According to the National Drought Mitigation Center, it can be hard to measure drought since it’s difficult to determine when it begins and ends.

The Palmer Drought Severity Index is one example of a widely used index that indicates prolonged and abnormal moisture deficiency or excess. The index is based on an equation that uses precipitation, temperature and soil moisture to calculate drought severity. The result is a number that indicates drought classifications.

Another drought measurement tool is The Standard Participation Index which “shows the actual precipitation compared to the probability of precipitation for various time frames.” A short term drought monitoring tool can be found in the Crop Moisture Index, which is used to monitor moisture in crop-producing regions.

The National Drought Mitigation Center has a drought monitor that is updated on a regular basis. Check that out to see where conditions stand in your region in the United States.

Climate patterns (like El Niño) affect global climate unpredictability. These patterns tend to reoccur periodically with enough frequency and with similar characteristics to offer long-range climate prediction.

Global climate change may cause some areas to receive less rain while others receive more. In addition, climate change may impact the intensity of rainfall, so that more rain falls over a shorter period of time. Climate change could also impact drought by increasing temperature, which could increase demand and evaporation.

• Phase 1 - Watch:
  5 to 10 percent shortage (voluntary reductions); During this stage of drought, your city government is likely to explain the drought situation, forecast future actions and suggest voluntary conservation actions.
• Phase 2 - Warning:
  10 to 20 percent shortage (voluntary or mandatory reductions; Voluntary conservation actions initiated in Phase 1 are ongoing. The city may require that consumers reduce their water consumption by 10-20 percent at this stage.
• Phase 3 - Emergency:
  20 to 35 percent shortage (mandatory reductions); In order to provide water for everyone, your city government may institute rationing programs by issuing fixed allotments based on per capita or per household data, such as lot size, past usage, or other data. They may also require all homes to have low-flow showerheads and toilet displacement devices or ultra-low flush toilets before granting an increased allotment. In addition, all municipal pools may be required to cover their pools to prevent evaporation.
• Phase 4 - Critical:
  35 to 50 percent shortage (mandatory reductions); All steps implemented in Phase 3 are likely to intensify, and local law enforcement will monitor for compliance. Outdoor water use may be cut completely.