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

Hundreds of millions of Americans remain affected by yet another week of sweltering heat as excessive heat watches are enacted across the U.S., from California to New York. 

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. But this July has been particularly stifling, breaking more than 2,400 temperature-highs in the last 30 days across the country. July is also on pace to become the hottest month ever recorded globally, and it’s due to a heat dome—a dome of high pressure that traps air and heat— in the U.S. 

Temperatures this week were particularly high in the American South and Southwest, where cities like Phoenix are projected to hit an average monthly temperature of more than 100 degrees. But Accuweather Meteorologist Joseph Bauer says that the current heat dome that is causing extreme heat will soon extend east, bringing high temperatures towards the lower Mississippi Valley and central U.S. 

The current heat dome has been impacting the country for more than a month, which experts say is unusual. “Usually heat domes break down after a week, maybe two weeks, but in this case, our heat dome is refusing to budge,” meteorologist Matthew Cappucci tells TIME. “[It started] in mid June, and really remained the standard across parts of the Southwest and now central U.S…and it's expanding in terms of longitude west to east.” 

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. However, Bauer says that one of the contributing factors for this month’s heat dome has to do with sea surface temperature anomalies, which is when the temperature of the ocean’s surface differs from its average measure. 

So far this year, the Southwest coast has seen below average sea surface temperatures. However, Bauer says that meteorologists have seen sea surface temperatures increase across the Northern Pacific—mainly off the coast of Washington, Oregon and British Columbia. Those temperatures create 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.

Will heat waves continue to occur? 

This weekend, excessive heat warnings are in effect in areas like New York City and Boston, with temperatures expected to rise above the 90s.  

Experts are unclear about how long heat waves will persist because pressure changes happen daily, though Cappucci tells TIME that he foresees heat waves continuing into August in the U.S. 

The National Weather Service’s Climate Prediction Center says that above normal temperatures are likely to take place in the Plains and across the South through August 6. A monthly temperature outlook by the Center, however, shows that temperatures will largely remain average across much of the U.S. come mid-August, though that is still to be seen. 

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.