Climate Overview Detailed Climate Perspectives . . . The warmest Winter season on record was December 1991-February 1992. 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 Winter 1998-99 as the second warmest such season since records began in 1895. The daily difference in temperature may be lower in areas where a single air mass remains dominant. The daily difference in temperature will be higher in areas that experience a greater frequency of frontal passages as cold arctic air moves southward and warmer, maritime air moves northward. This will happen under a variable circulation pattern, or along a stable storm track. The magnitude of this ratio is expressed by the size and color of the dot representing the value at each of approximately 250 First Order airport stations across the United States.