January Overview Detailed Climate Perspectives . Monthly Reports National Temperature Preliminary data for January 1999 indicate that temperature average across the contiguous United States was above the long-term mean ranking as the 13th warmest January since 1895. The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the temperature for each of 344 climate divisions in the U.S. by using their 1931-90 mean and standard deviation, then weighting these divisional values by area. These area-weighted values are then normalized over the period of record.
Positive values are warmer and negative values are cooler than the mean.
The preliminary national standardized temperature index ranked January 1999 as the 10th warmest such month on record. January 1999 was the 13th wettest such month since 1895, based on preliminary data. The national precipitation index expresses precipitation departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the annual precipitation in each of 344 climate divisions across the U.S. using the gamma distribution over the 1931-90 period. These gamma-standardized divisional values are then weighted by area and averaged to determine a national standardized value for each year.