Green shading indicates that daily variability in temperature was less than normal and may be a consequence of a dominant air mass. Red shading indicates that daily temperature variability was greater than normal reflecting a more frequent passage of differing air masses. 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 indicate warmer than the mean and negative values indicate cooler than the mean.
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. These national values are normalized over the period of record. Negative values are drier and positive values are wetter than the mean.