Is summer getting longer where you live? See how temperatures changed.

Summer heat isn’t just for the summer anymore.

In recent decades, sweat-inducing temperatures have been arriving earlier and ending later in the year. An analysis of U.S. weather data shared with The Washington Post shows which places are experiencing notably longer summer seasons than they were three decades ago. Temperatures are spiking to levels typically seen in June earlier than expected and lingering longer at the end of the season.

The analysis, conducted by climatologist Brian Brettschneider, examined the hottest days 90 days of the year from 1965 to 1994 and compared their frequency over 1995 to 2024. He found that the temperatures that used to kick off the hottest three months of the year are expanding beyond the calendar definition of summer.

In D.C., for example, the city’s Northwest area used to see summer temperatures — starting at 73 degrees — begin June 9 and end Sept. 6. Now, the city starts to have summer temperatures around June 5 and cools off near Sept. 11.

“What we think of as summer isn’t necessarily the same today as it was 30 years ago,” said Michael Allen, a climatologist at Towson University who was not involved in the analysis.

The southern United States has seen the most warming, gaining an extra two weeks overall of hot summer temperatures. Key West, Florida, for example, is experiencing summer temperatures for 39 days longer than three decades ago.

The Mid-Atlantic and Pacific Northwest have seen a moderate expansion. Summer temperatures are lasting about 9 days longer in D.C. Meanwhile, New York City and Seattle see about 6 more days than 30 years ago.

Cities along the coast of California have experienced more summer heat, too. Many coastal areas don’t typically see peak summer until the end of August because of local wind and sea surface temperature patterns, but a lot of locations have expanded their season. For example, San Francisco has turned up the heat for a whopping 42 days longer.

The Midwest has seen the least lengthening of the season, with a few areas shrinking their summer. For instance, much of Iowa has seen its hottest temperatures extend about 5 days longer than before. Whereas summer temperatures in Great Falls, Montana, are stopping four days earlier.

The rate of change over the past 30 years “is faster than anticipated,” said Yuping Guan, a physical oceanographer at the Chinese Academy of Sciences; he was not involved in this analysis. A few years ago, he and his team did a global study showing that summer temperatures have lasted a few more days each decade in the Northern Hemisphere. Their study used slightly different datasets but, he said, “the trends are consistent” and this analysis is more accurate for the United States.

The root of the prolonged summer heat is the overall increase in global temperatures. The recent increase in greenhouse gas emissions, through the burning of fossil fuels, traps heat in our atmosphere and raises our average global temperature. The past 10 years have been the hottest on record.

Factors such as latitude, topography, elevation and cyclical weather patterns like El Niño and La Niña also play a role in which areas are the warmest in general, but the lengthening of the summer season is primarily driven by global warming, Guan said. In Guan’s study from 2021, he found summer temperatures could last for six months in the Northern Hemisphere by the end of the century.

Overall, areas near the coastline are seeing the longest duration of summer heat because of their proximity to warming seas. The ocean absorbs 90 percent of the net heat in the atmosphere, which slowly transfers to nearby land. In the past six decades, the amount of ocean heat has been building to unprecedented levels.

“We know that the [latitude] of the sun isn’t changing necessarily,” said Allen of Towson University. “Most of the changes are caused by the warming of the planet because of greenhouse gases being burned.”

Cities also experience longer summers than rural areas in the same general location. For example, Los Angeles is enduring 23 more days of summer heat. But about 100 miles east, Joshua Tree is seeing around 15 more of its hottest days.

But rural communities are just as vulnerable as cities, if not more so, Allen said, because they may have fewer health-care resources or more residents who are aging.

As anyone sweating through the season in the United States can attest, the peak of summer is also getting warmer. An analysis from the nonprofit group Climate Central showed that summer has become about 2.6 degrees Fahrenheit warmer since 1970 in more than 200 major cities throughout the United States.

These longer, hotter seasons are challenging how our communities cope with summer. More air-conditioning can strain power grids, causing rolling blackouts. The prolonged heat can stress animals and plants, affecting crop production or the distribution of pests.

Allen said it’s important to be strategic in how urban and rural communities are built to withstand the changes.

But the first step to prepare for these longer summers, Allen said: Know they’re happening.