More than three-quarters of the United States is in the midst of a record-breaking heat wave.
The sweltering situation is caused by a phenomenon called a heat dome, during which hot, high-pressure air camps out over the continental United States. A heat dome “is really just sort of a colloquial term for a persistent and/or strong high-pressure system that occurs during the warm season, with the end result being a lot of heat,” says UCLA climate scientist Daniel Swain to Wired’s Matt Simon.
Some regions have seen days on end of unusually high temperatures. Muskegon, Michigan, saw nine straight days with highs over 90-degree Fahrenheit, a new record. Likewise, Buffalo, New York, had eight straight days over 90 degrees, also a record there, according to Weather’s Jonathan Erdman.
Parts of California, Nevada and Arizona are under excessive heat warnings from the National Weather Service, and Phoenix, Arizona, has had ten straight days of over 110-degree weather. The National Weather Service in Phoenix expects that temperatures will remain above normal for the foreseeable future, reports Matthew Cappucci for the Washington Post.
According to the National Oceanic and Atmospheric Administration, a heat dome occurs because of strong changes in water temperature in the Pacific Ocean during the previous winter. The warmth from the ocean heats the atmosphere above it, and that drives heat and humidity east, across the ocean.
In summer, the jet stream—the air current that separates cold Arctic air from the rest of the atmosphere—moves north and the hot air from the Pacific becomes trapped beneath it where it sinks to ground level above the continental U.S.
“So the same air that's maybe 80 degrees a few thousand feet up, you bring that same air—without adding any extra energy to it—down to the surface in a high-pressure system and it could be 90, 95, 100 degrees,” Swain tells Wired.
Then, tropical air from the Gulf of Mexico is also pulled into the dome, adding more heat and humidity, as Shannon Van Sant explained for NPR in 2019. The end result is a feedback loop where the heat wave causes itself to last longer. At first, some of the sun’s energy is put toward evaporating water, but after all of the moisture has been whisked away, the sun’s energy mostly heats the air. As Swain puts it to Wired, heat domes “start to feed off of themselves.”
Another sign of the extremity of the heat wave comes from the atmosphere’s height. When the air heats up, the atmosphere gets taller, per the Washington Post. In parts of the southwest, the atmosphere’s midpoint-by-mass is almost 500 feet higher than usual. Although that might sound minimal, the “threshold is very rarely seen on weather maps,” Cappucci, a meteorologist with the Post’s Capital Weather Gang, writes.
This event is far from America’s first heat dome. In the last decade, heat domes were reported in 2011 and then every year since 2016. The years from 2016 to 2019 are also all in the top five hottest years on record, according to NOAA.
Heat waves like these can be serious threats to public safety.
“The combination of heat and humidity can take its toll on someone who is outside and overdoing it,” National Weather Service meteorologist Richard Bann told the New York Times’ Farah Stockman in 2019. “It can be life-threatening.”
Wired reports that the dangers from high heat could compound with dangers from the COVID-19 pandemic, which is keeping people in homes that may not have air conditioning. Extreme heat caused the deaths of over 7,000 people between 1999 and 2010, according to the Centers for Disease Control and Prevention. The CDC has created specific guidelines for community cooling centers, which have the potential to become COVID-19 hotspots by bringing together many people in close proximity indoors.
“Already, we knew before Covid that one in three American households was struggling to pay their energy bills,” says Chandra Farley, who directs the Just Energy program at the Partnership for Southern Equity, to Wired. “We knew folks were already keeping their homes at uncomfortable temperatures for fear of running up their bills.”