Nearly six percent of the country was much warmer than normal while about one percent of the country averaged much cooler than normal. Five of the last six summer seasons have been above the long-term mean. The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's temperature value is standardized for each of 344 climate divisions in the U.S. by using their 1931-90 means and standard deviations, 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 also ranked Summer 1999 above the long term mean. 1999 average temperatures as a departure from 1961-1990 station normals. The overall pattern consisted of widespread warmth from the southern Plains to the Northeast, and generally below-normal average summer temperatures from the northern Rockies to southern California, with a mixed pattern in the transition area between these two. The Hawaiian stations averaged slightly cooler than normal, while Alaska was near to above normal. Summer 1999 was the first such season since 1988 to be much drier than normal.