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Index

Climate Extremes Index

The U.S. Climate Extremes Index (CEI) was proposed in 1995 as a framework for quantifying observed changes in climate within the contiguous United States. The CEI is based on a set of climate indicators: extremes in monthly mean maximum and minimum temperatures, heavy 1-day precipitation events, drought severity, the number of days with/without precipitation, and wind intensity of landfalling tropical cyclones.

A CEI value of 0 percent, the lower limit, indicates that no portion of the country was subject to any of the extremes of temperature or precipitation considered in the index. In contrast, a value of 100 percent would mean that the entire country had extreme conditions throughout the time period for each of the indicators, a virtually impossible scenario. Since we're dealing with the upper and lower tenth percentile as a definition of the extremes, and we're looking at the cold and warm (wet and dry) ends of the extremes, the long-term average expected percent area experiencing extremes is 20 percent. Therefore, observed CEI values of more than 20 percent indicate "more extreme" conditions than average, and CEI values less than 20 percent indicate "less extreme" conditions than average.

The CEI is evaluated for eight seasons: spring, summer, autumn, winter, annual, cold season, warm season, and hurricane season. Data and graphics for each season and indicator are updated at the beginning of the month. 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 (see figure). Recent percentages are similar to those found during the early 1900s for these same periods.

Data and graphics for the most current CEI and the individual indicators within it are available online at http://www.ncdc.noaa.gov/extremes/cei/.