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.
The number within the state represents that state s ranking for the given period, compared with all other such periods for that state for the 107 year period of record. For example, if a state has number 102, this means that out of 107 years, this stated period ranked 102 out of 107, or fifth warmest or wettest for that state. If a state has number 15, this means that out of 107 years, this stated period ranked 15 out of 107, or fifteenth coolest or driest. The First Difference Method is an approach developed by Peterson et al. For example, when creating a station's first difference series for mean February temperature, the station's February 1880 temperature is subtracted from the station's February 1881 temperature to create a February 1881 first difference value.