Another form of severe weather is drought, which is a prolonged period of persistently dry weather (that is, absence of precipitation).[88] Although droughts do not develop or progress as quickly as other forms of severe weather,[89] their effects can be just as deadly; in fact, droughts are classified and measured based upon these effects.[88] Droughts have a variety of severe effects; they can cause crops to fail,[89] and they can severely deplete water resources, sometimes interfering with human life.[88] A drought in the 1930s known as the Dust Bowl affected 50 million acres of farmland in the central United States.[88] In economic terms, they can cost many billions of dollars: a drought in the United States in 1988 caused over $40 billion in losses, exceeding the economic totals of Hurricane Andrew, the Great Flood of 1993, and the 1989 Loma Prieta earthquake.[89] In addition to the other severe effects, the dry conditions caused by droughts also significantly increase the risk of wildfires.