California-Great Basin

What is Drought?

A drought is a period of unusually persistant dry weather that persists long enough to cause serious problems such as crop damage and/or water supply shortages. The severity of the drought depends upon the degree of moisture deficiency, the duration, and the size and location of the affected area. There are actually four different ways that drought can be defined.

• Meteorological - a measure of departure of precipitation from normal. Due to climatic differences, what might be considered a drought in one location of the country may not be a drought in another location.
• Agricultural - refers to a situation where the amount of moisture in the soil no longer meets the needs of a particular crop.
• Hydrological - occurs when surface and subsurface water supplies are below normal.
• Socioeconomic - refers to the situation that occurs when physical water shortages begin to affect people.

For the continental U.S., the most extensive U.S. drought in the modern observational record occurred from 1933 to 1938, the "Dust Bowl" period. In July 1934, 80% of the U.S. was gripped by moderate or greater drought, and nearly two-thirds (63%) was experiencing severe to extreme drought. During 1953-1957, severe drought covered up to one half of the country.

Because of their widespread occurance, droughts often produce economic impacts exceeding $1 billion. The costliest drought on record was the 1988 drought, which devastated crops in the Corn Belt, causing direct crop losses of $15 billion and much larger additional indirect economic impacts.

There is nothing we can do to prevent droughts since they result from long-term shifts in storm tracks away from the affected region, or persistent wind patterns that reduce the flow of moisture into a region. Often, "blocking weather patterns" that feature persistent, stationary high-pressure regions over an affected area are observed with droughts.