In order to place each month and season into historical context, NCDC assigns ranks for each geographic area (division, state, region, etc.) based on how the temperature or precipitation value compares with other values throughout the entire record when sorted from highest to lowest value. In other words, the numeric rank value within the area represents the position or location of the sorted value throughout the historical record (1895-present). As a year is added to the inventory, the length of record increases. In 2010, NCDC had 116 years of records, thus the number 116 would represent the warmest or wettest rank; the number 1 would represent the coolest or driest rank. If a state has rank of 110, then it would be the seventh warmest or wettest on record for that time period. If a state rank has a value of 7, then that state ranked seventh out of 116 years, or seventh coolest or driest.
The "Below Normal", "Near Normal", and "Above Normal" shadings on the color maps represent the bottom, middle, and upper tercile (or three equal portions) of the distribution, respectively. The lowest and uppermost decile (or 10%) of the distribution are marked as "Much Below Normal" and "Much Above Normal", respectively. In other words, for a 116-year period (1895-2010), a rank of Much Above/Below Normal, would be in the top/bottom 12 on record. Below/Above Normal, would represent one of the 39 coolest/warmest or driest/wettest such periods on record. "Near Normal" would represent an average temperature or precipitation value that was not one of the 39 coolest/warmest or driest/wettest on record. For a 116-year period of record, "Near Normal" would represent a rank between 40 and 77.
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