From Texas clear to Georgia, from the Gulf Coast on up to the Canadian border, a mass of dangerous heat has started spreading like an atmospheric plague. In the days and perhaps even weeks ahead, a high-pressure system, known as a heat dome, will drive temperatures over 100 degrees Fahrenheit in some places, impacting some 160 million Americans. Extra-high humidity will make that weather even more perilous—while the thermometer may read 100, it might actually feel more like 110.
So what exactly is a heat dome, and why does it last so long? And what gives with all the extra moisture?
A heat dome is a self-reinforcing machine of misery. It’s a system of high-pressure air, which sinks from a few thousand feet up and compresses as it gets closer to the ground. When molecules in the air have less space, they bump into each other and heat up. “I think about it like a mosh pit,” said Shel Winkley, the weather and climate engagement specialist at the research group Climate Central. “Everybody’s moving around and bumping into each other, and it gets hotter.”
But these soaring temperatures aren’t happening on their own with this heat dome. The high pressure also discourages the formation of clouds, which typically need rising air. “There’s going to be very little in the way of cloudiness, so it’ll be a lot of sunshine which, in turn, will warm the atmosphere even more,” said AccuWeather senior meteorologist Tom Kines. “You’re just kind of trapping that hot air over one part of the country.”
In the beginning, a heat dome evaporates moisture in the soil, which provides a bit of cooling. But then, the evaporation will significantly raise humidity. (A major contributor during this month’s heat dome will be the swaths of corn crops across the central U.S., which could help raise humidity in states like Minnesota, Iowa, and Indiana above that of Florida.) This sort of high pressure system also grabs moisture from the Atlantic Ocean and Gulf of Mexico, which evaporate more water the hotter they get. And generally speaking, the warmer the atmosphere becomes, the more moisture it can hold. Once that moisture in the landscape is all gone, more heat accumulates — and more and more. A heat dome, then, essentially feeds off itself, potentially for weeks, a sort of giant blow drier pointed at the landscape.
On their own, temperatures soaring over 100 are bad enough for human health. Such high humidity makes it even harder for the human body to cool itself, because it’s harder for sweat to evaporate. Hence 100 degrees on the thermometer feeling more like 110. The elderly and very young can’t cool their bodies as efficiently, putting them at higher risk. Those with heart conditions are also vulnerable, because the human body tries to cool itself by pumping more blood. And those with outdoor jobs—construction workers, garbage collectors, delivery drivers on bikes or scooters—have little choice but to toil in the heat, with vanishingly few laws to protect them.
The humidity effect is especially pronounced in areas whose soils are soaked with recent rainfall, like central Texas, which earlier this month suffered catastrophic flooding. There’s the potential for “compound disasters” here: relief efforts in inundated areas like Kerr County now have to reckon with soaring temperatures as well. The Gulf of Mexico provided the moisture that made the flooding so bad, and now it’s providing additional humidity during the heat dome.
A heat dome gets all the more dangerous the longer it stagnates on the landscape. And unfortunately, climate change is making these sorts of heat waves longer and more intense. According to Climate Central, climate change made this heat dome at least five times more likely. “These temperatures aren’t necessarily impossible, but they’d be very hard to happen without a fingerprint of climate change,” Winkley said.
Summer nights are warming almost twice as fast as summer days, Winkley adds, which makes heat waves all the more dangerous. As this heat dome takes hold, nighttime low temperatures may go up 15 degrees above average. For those without air conditioning—or who can’t afford to run it even if they have AC—their homes will swelter through the night, the time when temperatures are supposed to come down and give respite. Without that, the stress builds and builds, especially for those vulnerable groups.
“When you look at this heat wave, yes, it is going to be uncomfortable during the day,” Winkley said. “But it’s especially those nighttime temperatures that are the big blinking red light that this is a climate change-boosted event.”
