100-degree days are becoming more common in the cities where most people live. These charts explain why.

By Christopher Hickey, CNN

Hot summer days in the United States have only been getting hotter. In the 25 largest US cities, days with highs breaking the 100-degree Fahrenheit barrier have become more common over the past 75 years, according to airport weather station data from the National Oceanic and Atmospheric Administration.

This year is no exception. Looking at the first seven months of each year, 2023 had the sixth-highest number of triple-digit highs in the 25 largest cities with available data since 1948, according to NOAA. Last year had the most days over 100 degrees Fahrenheit through the end of July in 75 years.

Experts told CNN the climate crisis is intensifying heat waves and pushing days that would normally top out in the upper-90s into the triple-digits. As the number of days with extreme heat become more common, that puts an increasing strain both on individuals – especially those who work outside or don’t have access to adequate shelter – and a city’s infrastructure.

“The climate has already changed. We’ve had increases in greenhouse gases for the past 150 years or more,” said David Easterling, chief of the Climate Assessments Section at NOAA’s National Centers for Environmental Information. “And what that translates to is that whether in Phoenix or Las Vegas, it’s just getting warmer there.”

Overall, people living in the 25 largest cities have put up with an average of three more annual days over 100 degrees over the past 20 years than between 1948 and 1980. The average number of triple-digit temperature days have become more frequent between those two periods in 17 of the 25 largest US cities with available data, according to NOAA.

Exactly how much more frequent triple-digit heat days have become varies widely across the country. Portland, Oregon, is seeing an additional day each year on average above 100 degrees since 2003 than it did between 1948 and 1980. Phoenix, on the other hand, is averaging 18 more triple-digit heat days between those same two periods.

The number of triple-digit days has risen the most for a handful of cities in the Southwest and South. Residents of Phoenix, Austin, El Paso and San Antonio have seen especially startling temperature changes over the past 20 years.

Between 1948 and 1980, there were 11 years when San Antonio didn’t see a single day above 100 degrees. Since 2003, though, the city has gone only one year without a day in the triple-digits.

In Phoenix, a year with under 90 days with temperatures above 100 degrees used to be fairly common – it happened in 16 of the years before 1980. That’s no longer the case. The last time the city saw fewer than 90 days in the 100s was in 1998.

There are always going to be local differences – the number of days over 100 degrees haven’t increased as much in Las Vegas as they have in Phoenix, Austin, El Paso or San Antonio, for example. But generally, temperatures go up significantly in areas where the soil and air are dry, Easterling said.

Air moisture, or humidity, regulates temperatures and prevents it from spiking to extreme levels. Phoenix, which is located in the Sonoran Desert, runs so hot in part because of its dry climate – it has averaged just 7.22 inches of rain per year since 1948, according to Phoenix Sky Harbor International Airport weather station data. That’s less than an inch per month.

But while humidity may moderate the air temperature, it can also make hot temperatures feel even hotter and put more pressure on the body, which struggles to cool itself in humid weather.

“It’s important for people to remember that impacts on people and infrastructure don’t start at 100 degrees,” said James Doss-Gollin, an assistant professor at Rice University’s Department of Civil and Environmental Engineering. “Things like humidity and air quality can have a big impact on how our bodies feel heat.”

Even relatively small increases in the number of days of triple-digit temperatures can have a dramatic effect on a population.

Seattle’s Sea-Tac International Airport, for instance, recorded the lowest average number of days above 100 degrees out of any of the 25 largest cities over both 1948-1980 and since 2003. Between 1948 and 1980, not a single day broke 100 degrees. Then the airport registered three triple-digit days in 2021 alone.

“Urban areas really have to start taking this seriously and begin to say, ‘How can we create a cooler environment?’” Easterling said. “Because we’re going to see temperatures increase as we continue to increase greenhouse gas. It’s a big issue.”

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