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While droughts tend to be less spectacular than other types of natural disasters, they actually affect more people in North America than any other type of natural hazard. In addition, droughts are more costly to the United States than any other type of disaster, with losses of $6 to $8 billion every year.
No region in North America is immune to droughts, and at least one region in the United States experiences drought in any given year.
A drought is a period of abnormally dry weather that lasts long enough to produce a serious imbalance in the water cycle. A drought is defined and measured in relation to normal climate conditions. In other words, a drought in Virginia could equate to wet conditions in Arizona. The specific factors that define droughts are rainfall amounts, vegetation conditions, agricultural productivity, soil moisture, reservoir levels, changes in river and stream flows and economic impacts. Droughts can last for many years and have devastating effects on agriculture, water supplies and the economy.
Dry weather is common, but when it happens over an extended period, it can become a serious problem. Dry weather causes a drought when there is a lack of normal precipitation for a season or more. This lowered precipitation results in a water shortage.
Wayne Palmer developed the Palmer Drought Severity Index, the most commonly used drought severity measurement tool, in the 1960s. This index uses temperature and rainfall information in a mathematical formula to determine dryness. The index is most effective over long periods of time, at least several months. Normal conditions are placed at zero, with dry conditions expressed as negative numbers and wet conditions as positive numbers.
