What makes a 'heat dome' and what does it mean?

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These unbearably hot and humid summer months that put millions of people across the globe at risk are typically made possible by phenomena known as heat domes.

Heat domes can make already-high temperatures even more extreme and prolonged, and they are worsening in severity and becoming more frequent as the planet warms.

Here's what to know.

Heat domes are essentially high-pressure systems hovering above a region that trap heat and humidity, experts say.

They result from the northward flow of warm air. That system sends air sinking, pressure increasing and temperatures rising.

“The concept of a heat dome really means that the air in this region is so warm, and we know that warm air expands," said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center. "It basically means that the layers of the atmosphere are bulging upward as well.”

Simply put, heat domes cause heat waves, experts say.

Heat impacted various regions of the world early in 2026.

The continental United States registered its most abnormally hot month in 132 years of records in March, with record-shattering extremes first in the Southwest and then across the rest of the nation. Heat scorched the French Open and scorched parts of India in May. Extreme heat has since been top of mind for the World Cup across the U.S., Canada and Mexico.

Starting mid-June, Europe has baked under unseasonable highs of around 40 degrees Celsius (104 Fahrenheit) in many places due to a heat dome.

Now, much of the Eastern U.S. is expected to be slammed by a long heat wave in the coming days, and already, much of the Southwest is experiencing temperatures around 100 degrees Fahrenheit (38 Celsius) this week and those temperatures could reach elsewhere by the Fourth of July holiday.

Science shows that as the planet warms — the result of humans burning coal, oil and gas — heat waves get worse, last longer and become more frequent.

“Heat waves like this are so directly connected to the climate crisis and climate change,” Francis said, "and it’s because of how we’ve been burning fossil fuels and cutting down forests for so long and increasing the concentration of heat-trapping gasses in the atmosphere.

“These kinds of heat waves and droughts and associated fires are all increasing just as we would expect them to in a warming world,” she said.

Experts say people need to stay hydrated when sweltering heat and humidity hit. Heat domes can make it difficult to fully recover from high temperatures at night, so finding ways to stay cool during the day and evening hours are important.

Be sure to avoid outdoor exercise in the heat of the day — and find shade, or if you can, access to air conditioning. Some cities provide resources and cooling centers to provide relief.

Cooling off in nearly pools or bodies of water can also help.

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Alexa St. John is an Associated Press climate reporter. Follow her on X: @alexa_stjohn. Reach her at [email protected].

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