Product Highlight: U.S. Climate Extremes Index

Annual U.S. Climate Extremes Index 1960-2013 Graph

The U.S. Climate Extremes Index or USCEI brings together several climate indicators to illustrate the occurrence of specific extreme events in the contiguous United States from 1910 to the present. These climate indicators include extremes in average monthly maximum and minimum temperatures, heavy one-day precipitation events, drought severity, the number of days with and without precipitation, and the wind intensity of tropical cyclones that make landfall in the country. The USCEI helps paint the picture of how often and how much of the country is dealing with extreme weather throughout time.

It does this by tracking the highest and lowest 10% of extremes in these climate indicators. One way to visualize how this works is by thinking of a football field. With temperatures for example, it would be as if the scientists took all of the average monthly maximum, or minimum, temperatures for a specific location and lined them up on a football field from the coolest at one goal line to the warmest at the other goal line. The USCEI incorporates those values that are within the 10-yard line on each end of the field. Scientists can then look at those values and see when most of the high and low extremes occurred, giving them an idea of how they are evolving over time.

So, how do you interpret the USCEI values? A USCEI value of 0% is the lower limit of the index, and it indicates that no portion of the country experienced any of the extremes included in the index. At the opposite end of the scale, a USCEI value of 100% would mean that the entire country had extreme conditions throughout the period for each of the indictors—virtually impossible. Because the USCEI looks at the upper and lower 10% of extremes, the average area of the country expected to be experiencing extremes is 20%. For that reason, observed USCEI values of more than 20% indicate “more extreme” conditions than average, while USCEI values less than 20% indicate “less extreme” conditions than average.

The USCEI is calculated for eight seasons: spring, summer, autumn, winter, annual, cold season, warm season, and hurricane season. Data and graphs for the most current seasons are updated at the beginning of each month. Scientists have also analyzed extremes across the nine standard regions: Northeast, Southeast, Ohio Valley, Upper Midwest, Northern Rockies and Plains, South, Southwest, Northwest, and West.

Learn more about the U.S. Climate Extremes Index or access its Graphs and Data.