U.S.: annual heat wave index 1895-2020

Based on the United States historical record, extended periods of very high temperatures were more frequent in the 1930s. In the United States, approximately 64 percent of the people lived in counties that experienced multiday heat waves in 2021.

Heat Wave Index in the United States between 1895 and 2020

Characteristic	Heat wave index
2020	0.13
2019	0.06
2018	0.12
2017	0.09
2016	0.07
2015	0.11
2014	0.03
2013	0.09
2012	0.26
2011	0.3
2010	0.15
2009	0.07
2008	0.03

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Release date

2021

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Region

United States

Survey time period

1895 to 2020

Supplementary notes

The U.S. Annual Heat Wave Index tracks the occurrence of heat wave conditions across the contiguous 48 states from 1895 to 2020. This index defines a heat wave as a period lasting at least four days with an average temperature that would only be expected to occur once every 10 years, based on the historical record. The index value for a given year depends on how often such severe heat waves occur and how widespread they are.

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