A drought is a kind of slow emergency. It never occurs suddenly. A drought results from long-continued dry weather and a lack of sufficient rain, whereby the dryness has become severe enough to cause concern over decreasing supplies of water, crop losses, and forest fire hazards. Droughts usually take place when the normal amount of rainfall of an area is 15 percent below what is usually received for a period of two to four weeks. The lack of rain is often accompanied by high temperatures, dry crumbling earth, hot, dry winds that blow away topsoil, and dried water bodies and wells. Drought causes considerable sickness and death to farm animals and livestock. When a region experiences a prolonged drought, farm products are in short supply, resulting in increased costs to consumers. In areas where farmers grow food only for their own subsistence, a lengthy drought can bring malnutrition, disease, and death due to food shortages. Widespread failure of crops because of a lack of water is one of the primary causes of famine.
The U.S. Weather Bureau distinguishes three categories of drought: (1) absolute drought, a period of at least fifteen days without measurable rainfall, (2) partial drought, a period of at least twenty-nine consecutive days in which the mean daily rainfall does not exceed 0.01 inches, (3) dry spell, a minimum of fifteen consecutive days during which less than 0.04 inches of rainfall is received.
Some geographers and climatologists define conditions as a drought when the annual precipitation is 75 percent below normal or monthly precipitation is 60 percent below normal. Others say that any amount of rainfall less than 85 percent of normal constitutes drought. Drought can be permanent, seasonal, irregular (variable rainfall), and invisible (insufficient rainfall to offset evaporation).