Much of the Western U.S. is experiencing extreme heat this week — with temperatures easily topping 100. Blame a condition known as a heat dome. But what is it?

If you want to visualize how a heat dome can trap a region in intensely hot weather, picture yourself making a grilled cheese sandwich.

“It almost acts like a lid on a pot,” the National Weather Service’s Alex Lamers tells NPR. He’s the operations branch chief at the Weather Prediction Center.

“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,” Lamers says. “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 that’s currently putting a hot lid on the Western U.S. will bring high temperatures that are 20 to 30 degrees hotter than normal for early June, the National Weather Service said. The forecast has both Phoenix and Las Vegas hitting 112 on Thursday, and the heat will stick around at night, falling only to the low 80s.

Here’s a guide to heat domes — and how you can stay safe:

How many people are affected by heat right now?

Some 20 million Americans, from California to Texas, were living under a federal excessive heat advisory as of Wednesday, with forecasters issuing alarms about the heat dome in the Western U.S.

Another 11 million people were under heat advisories.

“We are bracing already here in Tucson,” says Joellen Russell, a climate scientist and distinguished professor at the University of Arizona, where she also heads the Department of Geosciences.

“We’re going to be 108 or 109 starting [Thursday], and that will persist for three days, which is consistent with our definition of a heat wave, although we’ve been over 100 now for more than a week.”

What is a heat dome?

They’re generally caused by large high-pressure systems in the atmosphere. And they’re massive, linked to a ridge of high pressure. If the term “ridge” makes you think of a mountain feature, you need to go bigger: the term refers to curves in the jet stream.

“It would typically be several states,” Lamers said of the scale. “A third to half of the country.”

Russell says the jet stream behavior is “producing these stagnant high-pressure systems that are associated with extreme heat and drought,”

The jet stream normally spreads big storms — but right now, it’s too far north to bring moisture and potential relief to the Southwest.

“If it was blowing through Arizona, we’d be maybe even rainy,” Russell says. “But of course, it’s locked up there in northern Montana and instead where we’re experiencing warmer and warmer and warmer conditions.”

And when a heat dome sits over a large land area, Lamers says, a sort of feedback loop can take hold. High pressure typically means dry weather, which can help drive the heat even higher.

How much hotter does it get?

It depends on where you are, but if you’re enduring a heat dome, you’ll likely notice highs that are hotter than normal for that time of year. People in higher elevations, like among mountains, might avoid the worst effects.

“But it’s definitely easier to achieve those really hot temperatures in an absolute sense at lower elevations, in valleys” and urban areas, Lamers says.

How long do heat domes usually last?

They can last anywhere from a few days to a few weeks, Lamers says. “It really just depends on the overall weather pattern.”

In Tucson, a long stretch of extremely high temperatures is in the forecast.

“We’re expecting it to be up to 108 or 109 over the next few days,” Russell says. “It’ll come back down to be in the hundreds and then it’ll go back up to very, very hot. So there won’t be a real break over the next, say, 10 days.”

But it’s hard to predict how long a heat dome will persist, because they’re linked to the behavior of the jet stream.

“If the jet would swing south and break it up, that would be amazing — especially if it rained,” Russell says. But right now, she adds, the stream is likely to leave everyone from Idaho on south in very high heat.

Kids play in a splash pad at Riverview Park on Wednesday in Mesa, Ariz. Experts warn to make sure any outdoor play time includes plenty of water and shade — and breaks from the heat — during an extended heat wave.

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Does climate change cause heat domes?

The consensus from U.S. and international reports on climate change is that heat extremes are becoming more common, Lamers notes.

“You can get a heat dome or a configuration of the weather pattern that is similar to past cases,” he says. “But it’s going to be easier to achieve more extreme temperatures as a consequence of global warming.”

“We have more frequent heat waves now here in the U.S. and worldwide,” Russell says. “They are more frequent and they last longer.”

How can people stay safe in a heat dome?

“Heat is actually the deadliest weather-related hazard in the United States,” Lamers says, putting it above attention-grabbing events like tornadoes and hurricanes.

“What makes them so deadly,” Russell says, “is that people don’t understand that it’s not the first day of the heat wave that kills you. It’s the third or the fourth. You know, somebody decides at lunchtime to put their laundry out and it can be less than 15 or 20 minutes and you can have heatstroke.”

Russell, who is also a member of the nonpartisan group Science Moms, says she takes extra precautions with her kids, like getting a big, wide hat for her son, who’s a lifeguard. And because of the heat, the Tucson resident says, “we’re up at 5 to walk the dog so they don’t burn their little feet on the pavement.”

Russell says pools in her area will stay open longer, to give children a safe option to play outside. She’s also a big fan of sail shades, which protect kids at splash pads and help save water.

“We’ll also have cooling stations,” she says, and libraries will be open longer hours.

To stay safe during the day, people should take extreme heat seriously, staying hydrated and finding ways to break their exposure.

“And look out for other people in your life — neighbors, family, friends,” Lamers says. “Those community connections are really important to make sure people stay safe.”

Children, the elderly, and people with chronic conditions can be especially vulnerable. Lamers says anyone who isn’t getting a chance to cool off at night should pay special attention.

“We find this a lot, that actually the minimum temperatures have a pretty high correlation to fatality rates in these types of events,” Lamers says. “Basically, if Mother Nature isn’t allowing you to cool yourself down naturally at night with just the overnight temperatures, then it becomes really important that you find a way yourself to break that exposure to the heat.”

Track your heat risk in the U.S.

A new tool lets you see a map of dangerous heat across the contiguous country: The HeatRisk index comes from a collaboration by NOAA and the Centers for Disease Control and Prevention.

The color-coded map has a seven-day forecast, aiming to help people understand the health risks they could face.

Level 4, or magenta, is the most extreme category, signifying “rare and/or long-duration extreme heat with little to no overnight relief” that can affect anyone who doesn’t get enough hydration or cooling to mitigate the high temperatures.

But even if your area is orange, or at Level 2, you should still take care. In those conditions, most people who are sensitive to heat will be affected — especially if they don’t have ways to cool off, and stay hydrated.