Percent of Normal (Drought Index) One of the simplest measurements of rainfall for a location, a percent of “normal” figure is calculated by dividing the actual period precipitation by the normal precipitation, which is typically based on a historical average of a 30-year mean or average value. This figure is then multiplied by 100 percent to provide the drought index value. Normal precipitation for a specific location is always considered to be 100 percent, therefore, drought index numbers below 100 percent indicated various degrees of drought conditions. The percent of normal index, while easily understood and effective for assessing a single region or season, can be misunderstood as the concept of normal is a mathematical construct and does not necessarily correspond with what one should expect precipitation to be in any given year (i.e., the normal value may be merely an average of extreme conditions). Also see Drought Indexes (Indices).
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Author of the Water Words Dictionary source of text: Gary A. Horton
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