Droughts are caused by changes in land and sea surface temperatures, atmospheric circulation patterns and soil moisture content. A change in any one of these factors sets up a cyclical chain of events that can result in extreme climate conditions such as drought.
Unusual warming or cooling of sea surface temperatures can create a change in air temperature. This change of air temperature can affect the location of convection currents responsible for weather patterns, shifting warmer air over previously cooler locations. Heated air pulls moisture from the soil, allowing it to form clouds and return to the Earth as rain when it enters the cooler upper atmosphere. If weather patterns shift enough to create an area with little rainfall over a period of time, there is not enough moisture in the soil to draw up into the air to create clouds once the weather pattern shifts back to its former location. Moisture continues to be removed from the soil on a daily basis, and no rain clouds are able to form to replace the moisture.
Sustained dry periods can create droughts that cause economic distress across an area. Drought can impact crops, the water supply, stream water quality, recreation, hydropower generation, navigation and forest resources.