The Western U.S. Is Sweltering Under a ‘Heat Dome.’ What Does That Mean?

A stagnant high-pressure system over the region is trapping heat, exacerbating high temperatures and setting records

Aerial shot of children on a spray pool
Children play in a spray pool in Rio de Los Angeles State Park in Los Angeles, California, on Thursday, June 6. A heat wave led to record-setting temperatures across the western U.S. last week. Allen J. Schaben / Los Angeles Times via Getty Images

A heat wave in the western United States that started last week has been made even hotter by a “heat dome” sitting over the region.

This phenomenon occurs when a high-pressure system stays in the same place for days or weeks, trapping hot air beneath it, like a lid on a pot.

“If you’ve made grilled cheese in a pan and you put a lid on there, it melts the cheese faster, because the lid helps trap the heat and makes it a little bit warmer,” Alex Lamers, a meteorologist for the National Weather Service, tells NPR’s Bill Chappell. “It’s a similar concept here: You get a big high-pressure system in the upper parts of the atmosphere, and it allows that heat to build underneath over multiple days.”

The heat dome first contributed to an extreme, weeks-long heat wave in Mexico, and it has since spread to the southern U.S., according to New Scientist’s Michael Le Page. Temperatures surpassed 90 degrees in Mexico City on more than half the days in May. High temperatures and droughts have led to water shortages in the capital city this spring.

Last week, the extreme heat broke high temperature records across the southwestern U.S. for this time of year. Temperatures reached 111 degrees Fahrenheit in Las Vegas, Nevada; 113 degrees in Phoenix, Arizona; and 122 degrees in California’s Death Valley on Thursday, all daily records, per CNN’s Dalia Faheid and Robert Shackelford.

The heat dome is thought to be contributing to these record-breaking temperatures. Pressure systems usually move from west to east in the Northern Hemisphere, ushered along by the jet stream. But the jet stream can sometimes bend into a loop—shaped sort of like the Greek letter omega (Ω)—leading pressure systems to get blocked, according to the United Kingdom’s Royal Meteorological Society.

“If [the jet stream] was blowing through Arizona, we’d be maybe even rainy,” Joellen Russell, a climate scientist at the University of Arizona, tells NPR. “But of course, it’s locked up there in northern Montana and instead where we’re experiencing warmer and warmer and warmer conditions.”

Hot air underneath the heat dome gets heated even more—the dome pushes the heat air down and compresses it, which increases its temperature. As the ground warms, it dries up, making it easier to heat, too. The pressure system needs to get moving again to release the heat.

While warmer temperatures are becoming more common due to human-caused climate change, it’s still unclear whether global warming affects heat domes’ formation, per New Scientist.

One recent study, however, found that climate change “is actually favoring the jet stream behavior that produces these stagnant high-pressure systems and the extreme heat and drought associated with them,” Michael Mann, one of the study’s authors and an Earth scientist at the University of Pennsylvania, tells the Los Angeles Times’ Hayley Smith.

Currently, more than 24.8 million people in the U.S. are under active National Weather Service extreme heat advisories, watches and warnings. Six people thought to be migrants have died due to the heat wave along the U.S. border between southwestern Texas and New Mexico, per CNN.

Extreme heat is dangerous for human health—it’s the leading cause of weather-related deaths in the U.S. High temperatures pose heightened risks to people who work outside or don’t have air conditioning, people with chronic conditions and unhoused people, as well as infants, children and older adults. Around 1,220 people die in the U.S. each year due to extreme heat, according to the Centers for Disease Control and Prevention (CDC).

The CDC recommends that people avoid heat-related illness by staying in air-conditioned places, whether in your own home, a mall or a public library. You can also call your local health department to see if they have heat-relief shelters. The agency recommends taking cool showers and baths and limiting the use of the stove and oven.

According to the National Weather Service’s HeatRisk index, many states across the U.S. are predicted to experience “major” heat-related impacts this week. This level of heat affects people without adequate cooling or hydration and is expected to impact “health systems, heat-sensitive industries and infrastructure,” per the weather service.

Several areas of the Southwest are expected to face additional extreme heat this week. Temperatures are currently forecasted to reach 112 degrees Fahrenheit in Phoenix on Wednesday and Thursday; 110 degrees in Las Vegas on Wednesday; 108 degrees in Las Cruces, New Mexico, on Thursday; 106 degrees in El Paso, Texas, on Thursday; and 117 degrees in Death Valley on Tuesday and Wednesday.

Get the latest stories in your inbox every weekday.