Heatwaves are occurring more often than they used to in major cities across the United States, from an average of two heatwaves per year during the 1960s to more than six per year during the 2010s. The average heatwave season across 50 major cities is 47 days longer than it was in the 1960s. Of the 50 metropolitan areas in this indicator, 46 experienced a statistically significant increase in heatwave frequency; and 45 experienced significant increases in season length, between the 1960s and 2010s.
The most serious health impacts of a heatwave are often associated with high temperatures at night, which is when the daily minimum usually occurs. If the air temperature stays too warm at night, the body faces extra strain as the heart pumps harder to try to regulate body temperature.
Adjusting for humidity is important because when humidity is high, water does not evaporate as easily, so it is harder for the human body to cool off by sweating. That is why health warnings about extreme heat are often based on the “heat index,” which combines temperature and humidity.
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