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.
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.
Not only is heat wave season lengthening, but individual heat waves are also becoming longer. Nationally, an average heat wave has increased from three days long in the 1960s to four days long in the 2010s and 2020s.
New Orleans has had the greatest increase in heat wave length. There, heat waves now last 4.1 days longer on average than in the 1960s. Two other cities — Fort Worth, Texas and Salt Lake City — have had increases in heat wave length by at least three days.
While most of the studied cities have had longer heat wave seasons and longer individual heat waves, one-third of them have had more intense heat waves as well.
The EPA defines the heat wave threshold in each city is the average of the nine hottest days in July and August (the hottest months of the year) from 1981 to 2010. The agency measures heat wave intensity by determining the number of degrees by which the average heat wave surpasses the threshold.
In the 2020s, the average temperature of a heat wave has been 2.3°F above the heat wave threshold for each city. In the 1960s, it was 1.9°F.
Philadelphia has had the biggest increase in heat wave intensity, where heat waves are 1.51 degrees hotter than they were in the 1960s.
Across the country, increases in heat wave length, intensity, and season have made the summers longer and more dangerous.
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The EPA considers temperatures beyond the 85th percentile of historical July and August temperatures (1981-2020) for an individual city as outside of normal temperatures.
Statistical significance indicates that changes are due to a consistent trend and did not occur just due to chance.
Several large metropolitan areas did not have enough data, such as New York City, Houston, Minneapolis–St. Paul, and Denver. In some of these cases, the best available long-term weather station was relocated sometime between 1961 and 2021—for example, when a new airport opened.
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