Because of regional differences in the amount of precipitation considered normal, it is not possible to define and measure drought in any one way. In general, however, a drought can be thought of as any period of time during which precipitation (the amount of water from rain or snow) is below normal for the area for a certain period of time. The longer the period of below normal precipitation the worse the drought. Scientists and others studying droughts use a number of different "drought indices" to measure the severity of droughts. More information on various indices can be found here.