The U.S. Climate Extremes Index documents the area of the contiguous United States (or a region therein) that experienced extreme conditions (as defined by the index) during various time periods. The index is calculated as the arithmetic average of values from a specified set of extreme events, weighted by the percent area that experienced the defined conditions.

A value of 0% for the Climate Extremes Index indicates that no portion of the country experienced extremes considered in the index during the selected period of record. In contrast, a value of 100% would mean that the entire country experienced extreme conditions throughout the period of record for all of the indicators, a virtually impossible scenario. The long-term variation or change of this index represents the tendency for extremes of climate to either decrease, increase, or remain the same.