What to Know About Heat Domes—and How Long They Last

More than a hundred million Americans are set to be affected by what experts are calling the first “significant heat wave of the season,” as excessive heat watches expand from the Midwest to the Northeast, and could last from this week into next.

For residents across metro areas, including Boston, Chicago, Detroit, Cleveland, Philadelphia, and New York City, it's not just the increased temperatures that will stifle them—but rather, the duration of the sweltering heat through next week that can be dangerous to many. The wave is set to break or tie multiple records across the East.

 “The longevity of the dangerous heat forecasted for some locations has not been experienced in decades,” the National Weather Service Weather Prediction Center said Sunday.   

The peak of the heat wave for most of the affected area will arrive on Thursday, and heat is expected to reach dangerous highs in the Northeast, where temperatures could reach nearly 25 degrees above normal.

Despite 2023 making history as the hottest year on record, experts have warned that this year could surpass even those record-highs. Last week, nations across the Middle East and Europe endured their own heat waves, making the U.S. just the latest country to feel the burden of summer and climate change. Heat waves are becoming more common in major U.S. cities, the United States Environmental Protection Agency reports, happening an average of six times a year. 

Extensive heat waves that took place last July and broke more than 2,400 temperature-highs, were due to a heat dome—a dome of high pressure that traps air and heat. A heat dome is also contributing to the heat waves arriving this week.

“Usually heat domes break down after a week, maybe two weeks,” meteorologist Matthew Cappucci tells TIME. 

Though sometimes heat domes can “refuse to budge,” bringing in days that are hotter and hotter over time. 

Here’s what to know. 

What is a heat dome? 

Record-high temperatures have been brought forward by the current heat dome in the U.S. Heat domes, according to Gabriel A. Vecchi, professor of Geosciences at Princeton University, refer to an area of very high pressure in the atmosphere that extends a few miles upwards and traps hot ocean air. 

Heat domes and heat waves happen at the same time, Vecchi says, though a heat dome intensifies heat waves and causes them to persist. That’s because heat domes squash cloud coverage, rain, and deflect inclement weather, Capucci says. “Basically we just get the sun pouring down sunshine unimpeded, baking the ground, and no real cloud cover or moisture to stave off our temperatures.”

What causes heat domes? 

Heat domes can be caused by several factors. In a July 2023 interview with TIME, Accuweather Meteorologist Joseph Bauer said that one of the contributing factors for last July’s heat dome had to do with sea surface temperature anomalies, which is when the temperature of the ocean’s surface differs from its average measure. 

Bauer said that meteorologists saw sea surface temperatures increase across the Northern Pacific—mainly off the coast of Washington, Oregon and British Columbia—last year. Those temperatures created the perfect atmosphere for domes of high atmospheric pressure to build, Bauer says. 

Heat domes can usually last for days or weeks and usually affect large parts of the U.S. at the same time. Current warnings by the NWS say that local heat indexes—what the temperature feels like to the human body when humidity and air temperature are taken into account—could reach up to 105 degrees Fahrenheit in areas from the Ohio Valley to the Northeast. Overnight temperatures may only drop to the mid-70s. 

Will heat waves continue to occur? 

It's hard to predict how long heat waves will persist because pressure changes happen daily, Cappucci tells TIME. 

Experts say heat waves are likely to happen more frequently and more intensely in the upcoming decades. “Because global warming is almost certainly going to continue over the next century and enhance even if we reduce greenhouse gasses, there's going to continue to be an enhanced likelihood of heat waves in the northern hemisphere,” says Vecchi.