What is a heat dome?

With much of the Midwest and Northeast burning, or about to burn, in extreme summer heat this week, forecasters are talking about heat waves and heat domes.

Both mean it’s very hot, and people will hear those terms a lot more as the world warms. What is the difference?

Here’s what you should know:

What is a heat dome?

We can think of a heat dome as what is happening in the atmosphere. A heat wave is how it affects people on Earth, said Ken Kunkel, a research professor of atmospheric sciences at North Carolina State University.

When a high-pressure system develops in the upper atmosphere, it causes the air below to sink and become compressed. That raises temperatures in the lower atmosphere.

As the hot air expands, it creates a bulging dome.

The boundaries of this week’s heat dome are not well defined, according to Kunkel, but the National Weather Service (NWS) has said the most extreme heat is expected in the Ohio Valley and Northeast.

The eastern heat dome follows another earlier than usual one this month in the southwest. Last year, there were 645 heat-related deaths in Phoenix.

What is a heatwave?

A heat wave is defined by the intensity of the heat, how long it lasts and where it occurs, said Jeff Masters, a meteorologist with Yale Climate Connections.

In general, several days of temperatures above 90 degrees in Texas “isn’t a big deal,” Masters said. But further north, this week is forecast to be 32 and above in the Midwest and Northeast, with heat indexes of 38 degrees or higher.

“The population is simply not conditioned to that type of heat,” he said.

The National Weather Service said some areas will likely hit daily records, and the heat wave will last all week and into the weekend in some places.

The combination of clear skies and the sun’s higher angle in summer can result in high heat index readings, a measure of temperature combined with humidity. Humidity makes the weather seem hotter because the body cools down by sweating and has to work harder when the air is already humid.

The Detroit area will be above 32 degrees Celsius, with a heat index of around 38 degrees in some urban areas over the next few days. The normal high temperature for this time of year in Detroit is 26 degrees. Specifically, the normal high for June 18 is 27 degrees, meteorologist Brian Cromwell said.

Chicago broke a temperature record from 1957 with a high of 36.1 degrees Celsius. The heat would continue this week and the wind chill would sometimes be around 37.7, according to a post on the X social network from the NWS Chicago office.

In Cincinnati, Ohio, Tuesday’s high was around 36 degrees, but it felt like 40, according to the weather service. The intense heat will continue through the weekend.

Albany, New York, will see temperatures of 35 degrees or higher Tuesday through Thursday, when it will reach a high of 36, with heat indices as high as 38 or higher, according to the weather service.

New York Governor Kathy Hochul announced that she had activated the National Guard to assist in any heat emergencies that arise in the coming days.

Last year saw the largest number of heat waves in the United States, consisting of unusually hot weather that lasted more than two days, since 1936. Officials again warned residents to take precautions.

Who is under the heat dome?

The heat dome will affect a wide swath of the eastern half of the country, from roughly the Great Plains states to Maine.

Some places could experience the highest temperatures on record in any month, Masters said. A new study found that climate change is causing giant heat waves to move more slowly and affect more people for longer, with higher temperatures over larger areas.

Nearly 77 million people in the United States were under extreme heat alerts on Tuesday.

Another excessive heat warning, caused by a heat dome, is expected in the Phoenix area on Thursday and Friday, when highs could reach 45.5 and 47 degrees, respectively, said National Weather Service meteorologist Ted Whittock. Meanwhile, the maximum predicted for this Tuesday of 40.5 degrees is normal for this time of year.

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