A heat dome occurs when a persistent region of high pressure traps heat over an area. The heating dome can span multiple states and persist for days or even weeks, leaving people, crops, and animals below to suffer from hot, stagnant air that can feel like an oven.
Typically, heated domes are tied to the behavior of the jet streama band of fast winds high in the atmosphere that generally runs from west to east.
Normally the jet stream has a wavy, sinuous pattern north then south and again north. As these meanders of the jet stream become larger, they move slower and can become stationary. This is when heat domes can appear.
As the jet stream swings far north, air pools and sinks. The the air warms up as it descends, and the descending air also keeps the sky clear as it lowers the humidity. This allows the sun to create increasingly hot conditions near the ground.
If the air near the ground passes over the mountains and descends, it can even hotter. This downward warming played a significant role in the extremely hot temperatures in the Pacific Northwest during a heat dome event in 2021when Washington set a state record of 120 degrees Fahrenheit (49 Celsius) and temperatures reached 121 F in British Columbia, Canada, surpassing the previous Canadian record of 8 degrees F (4 C).
The human impact
Heat domes normally persist for several days at a given location, but they can last longer. They can also move around, influencing neighboring areas over a week or two. The heat dome involved in the June 2022 U.S. heat wave has moved eastward over time.
On rare occasions, the heat dome may be more persistent. It happened in the southern plains in 1980, when as many as 10,000 people died during weeks of scorching summer heat. This also happened in much of the United States during the Dust Bowl years of the 1930s.
A heated dome can have a serious impact on people, as the stagnant climate regime that allows it to exist typically results in low winds and increased humidity. These two factors aggravate the feeling of heat – and become more dangerous – because the human body is not so cooled by perspiration.
The heat index, a combination of heat and humidity, is often used to express this danger by indicating what the temperature will feel like to most people. High humidity also reduces the amount of cooling at night. Hot nights can prevent people without air conditioning from cooling off, increasing the risk of heat-related illnesses and death. With global warming, the temperatures are already highertoo.
One of the worst recent examples of heating dome impacts with high temperatures and humidity in the United States has occurred in the summer of 1995when we estimate 739 people died in the Chicago area over five days.