Over 75 million people across the United States, particularly in the Midwest and East Coast, are under extreme heat alerts even before summer officially starts this Thursday.

The Midwest has been experiencing intense heat with temperatures expected to exceed 37.8 degrees Celsius (100 degrees Fahrenheit). Meteorologists explain this extreme heat is due to high humidity and trapped hot air, a phenomenon known as heat domes.

So what exactly is a “heat dome” and how does it relate to a heatwave?

What is a heat dome?

A heat dome is a weather formation where high atmospheric pressure traps hot air beneath, similar to how a lid traps air in a pot. This pressure creates a dome of hot air, preventing cooler weather systems from passing through.

This generates prolonged periods of excessively high temperatures, contributing to heatwaves.

Are heat domes driving heatwaves in the US?

This week, heat domes will push temperatures 20-30 degrees Fahrenheit (11-16.5 degrees Celsius) above the average for June, as stated by the National Weather Service. A heatwave typically occurs when an area experiences at least two or more consecutive days of unusually high temperatures for that region.

For instance, a heatwave in the northeastern US is characterized by temperatures reaching 32.2 degrees Celsius (90 degrees Fahrenheit) or higher for three consecutive days. Heat domes are more frequent in the southern and western US, regions already prone to hot weather.

The term "heat dome" gained widespread usage only in recent years, although they have affected the US for decades. The American Meteorological Society added "heat dome" to its glossary in March 2022.

The frequency of heatwaves has been increasing in the US. Major cities have seen heatwaves rise from about two per year in the 1960s to six per year since the 2010s, according to an Environmental Protection Agency report. Additionally, heatwaves in many cities have become more intense and longer since the 1960s.

And beyond the US?

Heat domes have affected other countries as well. Last month in Mexico, heat domes contributed to a heatwave that claimed at least 22 lives and saw monkeys succumbing to suspected dehydration.

In August 2023, France experienced its hottest late summer days since 1947 due to a heat dome, with temperatures averaging 27.8 degrees Celsius (82 degrees Fahrenheit) nationwide and exceeding 40 degrees Celsius (104 degrees Fahrenheit) in southern regions.

What to do during a heatwave?

Health authorities globally advise taking precautions during a heatwave, such as staying hydrated, avoiding strenuous outdoor activities, and staying indoors during peak heat hours. Additional recommendations include placing a damp cloth on one’s head, neck, and face for cooling and avoiding high-protein foods to reduce dehydration.

Energy consumption should also be monitored. During last year’s heatwave in the US, residents of Texas were asked to conserve energy during peak hours from 2 pm to 10 pm to manage high demand.

How long does a heat dome last?

A heat dome can persist for several days to a few weeks, sometimes lasting over two weeks. The National Weather Service’s heat advisories are effective until 8 pm EDT (12:00 GMT) on Thursday or Friday, depending on the location.

Post this period, temperatures will gradually decrease but stay above normal for the subsequent week. The duration of a heat dome depends on the strength and persistence of the high-pressure system that caused it.

The National Weather Service predicts some of the most at-risk areas for extreme heat include Chicago and parts of the Midwest, such as Iowa, Wisconsin, and Indiana. Residents in these areas should prepare for prolonged and severe heat with little overnight relief.

Cities like New York and Pittsburgh are also at risk, with significant effects on those without adequate cooling during peak heat on Thursday and Friday.