Droughts can be caused by a number of things. The most important drought cause is related to how much water vapor is in the atmosphere because water vapor in the atmosphere is what causes precipitation. When there are moist, low pressure systems, precipitation, such as rain, hail, sleet, and snow can occur. If the presence of dry, high pressure is above average, there will be less moisture available to create precipitation. This then results in a water deficit in areas they move over.
This can also occur when air masses are shifted by winds, resulting in dry, warm, continental air moving over an area instead of moist, cooler, oceanic air masses. El Nino, which affects the temperature of the ocean's water, also impacts levels of precipitation because during the years in which the temperature cycle is present, the air masses can be shifted above the ocean, often leading to places that are typically wet, dry and making places that are typically dry, wet.