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Climatological Rankings
In order to place each month and season into historical context, NCEI assigns ranks for each geographic area (division, state, region, etc.) within the contiguous U.S. based on how the temperature or precipitation value compares with other values throughout the entire record when sorted from lowest to highest 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. As of 2019, NCEI has 125 years of records. Thus a rank of 125 would represent the warmest or wettest on record; a rank of 1 would represent the coolest or driest on record. If a state has a rank of 109, then it would be the 17ᵗʰ warmest or wettest on record. If a state rank has a value of 17, then that state ranked 17ᵗʰ out of 125 years, or 17ᵗʰ 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 125-year period (1895-2019), a rank of Much Above/Below Normal, would be in the top/bottom 13 on record. Below/Above Normal, would represent one of the 42 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 42 coolest/warmest or driest/wettest on record. For a 125-year period of record, "Near Normal" would represent a rank between 43 and 83.
For Alaska, the historical record begins in 1925 and thus, rank values are different than those used for the contiguous U.S. With 95 years in the record (1925-2019), a rank of Much Above/Below Normal would be in the top/bottom 10 on record. Below/Above Normal would represent one of the 32 coolest/warmest or driest/wettest such periods on record. "Near Normal" would represent a rank between 33 and 63.