Overview Detailed Reports and Images . . . Ten out of the last 13 years have averaged near to much warmer than the long-term mean. 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 national standardized temperature index ranked 1998 as the second warmest year on record. Preliminary data indicate that 1998 was the fifth wettest year on record for the United States. Each of the last nine years has averaged near to much wetter than the long-term 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.