What is a heat dome?

Summer often brings with it trips to the beach, ice cream, and fun in the sun. But it can also bring some very unpleasant–and downright dangerous–weather. Namely, extreme heat and stubborn pockets of high pressure and hot air colloquially called heat domes. 

What is a heat dome?

During a heat dome, hot air is trapped over an area of land for an extended period of time, similar to how a lid on a boiling pot of water keeps the heat inside of the vessel. Having such high temperatures over one area for a long strength of time causes dangerous and extreme heat. 

“A heat dome is a relatively new term in the lexicon, referring to a large area of warm air, a very warm air mass that’s associated with high pressure,” meteorologist and climatologist Eric Kelsey tells Popular Science. “They get a lot of attention because they often stick around in a region for a long enough period of time to cause significant health impacts to humans and other species too.”

Normally, the jet stream moves in a wave-like pattern. It travels north, south, and then back north again. If these blips in the jet stream become larger, they will slow down and become stationary. A high-pressure system will then stall in the upper levels of the atmosphere, trapping the air. These heat domes primarily form over large land masses during the summer months.

“In the summertime, the jet stream shifts poleward, or since we’re in the northern hemisphere, northward. When that happens, the jet stream and the associated weather patterns aren’t really moving much,” explains Kelsey. “This allows the high pressure that’s south of the jet stream and the air mass associated with it to warm up.”

The sun will also intensify this heat. There are often little to no clouds present due to the high-pressure system in place, so the sun’s radiation directly hits the Earth below. 

The opposite of a heat dome is the polar vortex, when cold Arctic air plunges southward and stalls.

What role does humidity play?

The old moniker “it’s not the heat, it’s the humidity” holds true with heat domes as well. In the United States, warm water from the Gulf of Mexico and parts of the Atlantic Ocean will evaporate water into the air during the summer.

“Usually with these heat domes, the surface high pressure is located somewhere around or just off the East Coast. That’s why we call it the Bermuda High,” says Kelsey. “Around the western side of the high pressure area are south winds that bring this moisture from the Gulf of Mexico northward across the Plains and across the Ohio River Valley.”  

[ Related: Almost every place on Earth was affected by extreme temperatures this summer. ]

On the West Coast, the Pacific Ocean and Gulf of California off of Mexico can have a similar effect on high-pressure systems. 

On either side of the country, this added water vapor can make the air more humid. That combination of heat and humidity is dangerous for people’s health and can cause heat related illnesses. 

What makes heat domes move?

The answer is pretty simple: wind and low-pressure systems.

“When you have the jet stream dipping southward and making a U-shape, we call it a trough. Where it goes upward is a ridge,” says Kelsey. “The ridge is where there’s warm air that has moved northward, and in the trough is where cold air has moved southward.”

When the jet stream dips southwards towards the equator, that movement can generate a low-pressure system and help move the heat dome further east and out into the ocean. Once it is over the ocean, the surface temperatures are cooler and will help bring the temperature down.  

Are heat domes increasing due to climate change?

Not surprisingly, the answer is yes. 

“We’re seeing them occur more frequently and more intensely,” says Kelsey. “Heat domes are certainly more intense when they happen because the jet stream is shifting further north in the northern hemisphere on average, especially in the summer.”

The record-breaking heat domes in the southwestern United States in 2023 were considered “virtually impossible” without climate change. Already this year, parts of Europe and China are seeing temperatures soar above 100 degrees Fahrenheit. Australia also saw its hottest 12-month period on record. 

How to stay safe in extreme heat

When the temperatures rise, water consumption should rise with it. The heat and humidity will make your body sweat more in an attempt to cool off, so it is important to replace that good old H20 as much as possible. Also, try to do any outdoor activities like gardening or running in the early morning and evening hours, to avoid the hottest times of the day. If you’re a non-human animal, “splooting” can also help. 

Seeking out air conditioning whether at home or a local cooling center is also critical during extended periods of extreme heat. 

[ Related: 5 ways to stay cool without blasting the AC. ]

“Usually after two or three days of extended extreme heat, heat illnesses really start to set in, because the body hasn’t had a chance in 72 hours to cool off,” says Kelsey. 

Cooling off with air conditioning when possible is particularly important if you live in a city. The urban heat island effect can raise the temperatures compared to suburban or rural areas with more tree cover and green space in general. 

“You are going to have possibly a five to six degree temperature difference, and that can be absolutely huge in terms of comfort and your health,” says Kelsey.

Ceiling, desk, and floor fans can also help move the air around and create a breeze. However, using a fan is helpful only up to a certain point.

“Fans can actually cause you to cook faster. They can help when the temperature is not too extreme, but if the air temperature is greater than 99 degrees with low relative humidity and your body needs to stay at 98 degrees, just blowing hotter air than your body temperature means you’re cooking yourself faster than without the fan,” says Kelsey. “Your body sweats less in hot, dry weather to conserve water and therefore, there isn’t enough evaporation to offset the heat.”

However, fans can be effective in hot and humid conditions, as your body will sweat enough to cool from evaporation.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.

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