The percentage areas of the contiguous United States are computed based on the climate division data set. Those climate divisions having the monthly average temperature in the top ten percent (> 90th percentile) of their historical distribution are very warm and those in the bottom ten percent (< 10th percentile) are very cold Likewise, those divisions having the monthly total precipitation in the top ten percent are very wet and those in the bottom ten percent are very dry.
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http://www.ncdc.noaa.gov/temp-and-precip/uspa/index.php