The Climate Extremes Index (CEI) was first presented in 1995 as a framework for quantifying observed changes in climate within the contiguous United States. The CEI is based on an aggregate set of conventional climate extreme indicators which now includes extremes in land falling tropical storm and hurricane wind intensity. Originally, the CEI was calculated on an annual basis, and now the revised CEI is evaluated for eight standard seasons: spring, summer, autumn, winter, annual, cold season, warm season, and hurricane season. Additional temperature and precipitation stations have been added to the analysis to improve spatial coverage without compromising completeness of data. Near real-time data have also been incorporated into the index, which allow the CEI to be calculated operationally on a seasonal basis.

Revised CEI results indicate that for the annual, summer, warm and hurricane seasons, the percent of the contiguous United States experiencing extreme conditions has been generally increasing since the early 1970s. Recent percentages are similar to those found during the early 1900s for these same periods. Seasons with a highly variable signal over time include the autumn, winter and cold seasons.