The U.S. Drought Monitor established a drought scale much like those that rate hurricanes and tornadoes. The "D-scale" speaks to the "unusualness" of a drought episode. Over the long run, D1 conditions are expected to occur about 10 to 20 percent of the time. D4 is much rarer, expected less than 2 percent of the time. For more detailed information about drought definitions, please visit the U.S. Drought Monitor.
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http://www.ncdc.noaa.gov/climate-monitoring/dyk.php