FENTON, Mich. — With much of the Midwest and the Northeast broiling — or about to broil — in extreme summer heat this week, meteorologists are talking about heat waves and heat domes.

Both mean it’s really hot — and people will hear those terms a lot more as the world heats up. What’s the difference?

Here’s what to know.

What’s a heat dome?

It’s helpful to think of a heat dome as what’s happening in the atmosphere. A heat wave is how that affects people on the ground, 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 it to sink and compress. That raises temperatures in the lower atmosphere.

Because hot air expands, it creates a bulging dome.

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

What is a heat wave?

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

In general, several days of 90-plus degree temperatures in Texas are “no big deal,” Masters said. But farther north, it is expected to be in the mid- to high-90s over the Midwest and Northeast, with heat indices of 100 degrees or greater.

“The population’s just not conditioned to that sort of heat,” he said.

The National Weather Service said that some areas likely will reach daily records, with the heat wave lasting all week and into the weekend in some places.

The combination of clear skies and the higher summertime angle of the sun can result in high heat index readings, a measure of temperature combined with humidity.

Humidity makes the weather feel hotter because the body cools itself by sweating and has to work harder when the air’s already moist.

The Detroit area will be in the mid-90s, with a heat index around 100 degrees in some urban areas for the next few days. The normal high temperature for this time of year in Detroit is in the low 80s.

Albany, New York, will see temperatures of 95 degrees or hotter through Thursday, when it will peak at 97 degrees, with heat indexes at 100 degrees or more, the weather service forecasted.

New York Gov. Kathy Hochul said Tuesday that she has activated the National Guard to assist in any heat emergencies.

The U.S. last year experienced the most heat waves since 1936, experts said.

An Associated Press analysis of Centers for Disease Control and Prevention data found that the excessive heat contributed to more than 2,300 U.S. deaths, the highest number in 45 years of records.

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