Which cities are experiencing longer and more intense heat waves?

Since the 1960s, heat waves across the US have consistently become more frequent, lasted longer, and been more intense.

The Environmental Protection Agency (EPA) defines heat waves as periods of two or more days where the weather is “abnormally and uncomfortably hot” and “outside the historical averages for a given area.” Heat waves can lead to crop damage, increased wildfires, illness, and even death from heat stroke.

The EPA has tracked the heat waves of 49 metropolitan areas in the US since the 1960s. Among these, 43 have had statistically significant increases in the length of their heat wave seasons, 27 have had increases in the length of heat waves, and 16 have had increases in heat wave intensity.

Where is heat wave season lasting longer?

Heat wave seasons, defined as the number of days between the first and last heat wave of a year, are growing longer for US cities. In the 1960s, heat wave seasons lasted around 24 days on average. In the 2020s, that number reached 73 days. Early spring or late fall heat waves can be dangerous, as people might be surprised and not adequately prepared.

Among the cities studied by the EPA, San Francisco has had the greatest increase in heat wave season length. On average, heat waves there lasted over three months longer than they did in the 1960s, a 111 day increase. Heat wave seasons also increased by more than 90 days in New Orleans and Tampa, Florida.