A heat dome is a large area of high atmospheric pressure that traps hot air over a region for days or even weeks, causing temperatures to climb well above normal. Think of it like an invisible lid sitting on top of the atmosphere: air sinks beneath it, compresses, heats up, and has nowhere to go. The result is prolonged, dangerous heat with little relief, even at night.
How a Heat Dome Forms
Heat domes begin with a strong high-pressure system that develops in the middle layers of the atmosphere, roughly 10,000 to 25,000 feet above the ground. This high-pressure zone acts as a cap. Beneath it, air sinks toward the surface. As that air descends, the increasing atmospheric pressure compresses it, and compressed air warms up. This process happens without the air absorbing heat from any external source; the compression alone raises the temperature.
At the same time, the high pressure suppresses the formation of clouds and rain. With clear skies overhead, the sun heats the ground relentlessly, and the trapped air near the surface keeps getting hotter. Warm surface air beneath a heat dome can persist for days because the airflow aloft is often calm and stagnant, with little wind to push the system along.
Why Heat Domes Get Stuck in Place
The jet stream, a fast-moving river of air high in the atmosphere, normally pushes weather systems across a continent. But sometimes the jet stream develops a large, persistent bend that wraps around a high-pressure zone in a shape resembling the Greek letter omega (Ω). Meteorologists call this an omega block, and it effectively anchors the high-pressure system in one spot. Weather systems approaching from the west are forced to detour around the block, leaving the region underneath baking in heat with no incoming storms to break the pattern.
An omega block can hold for a week or longer. During the 2023 heatwave over the southwestern United States and Mexico, a blocking high-pressure system persisted for more than six weeks.
The Role of Ocean Temperatures
What happens thousands of miles away in the ocean can determine whether a heat dome forms and how long it lasts. NOAA scientists found that record-warm sea surface temperatures in the North Atlantic were the dominant factor behind the extended 2023 southwestern U.S. heatwave. Warm ocean water altered large-scale wind patterns, which helped build and sustain the high-pressure system over land.
A developing El Niño in the Pacific played a smaller but significant supporting role. When both the Atlantic and Pacific were unusually warm simultaneously, the effect was dramatic: the combination doubled the number of regional heatwaves, tripled the total heat-wave days, and increased the duration of individual events by about 50%. Ocean conditions don’t just set the stage for heat domes; they can amplify them considerably.
Heat Dome vs. Heat Wave
Every heat dome produces a heat wave, but not every heat wave involves a heat dome. A heat wave is simply a period of abnormally high temperatures, however it happens. A heat dome describes a specific atmospheric setup: a strong, stagnant upper-level high-pressure system that traps and intensifies heat over a defined area.
Some meteorologists are skeptical of the term. The atmosphere beneath a high-pressure system doesn’t always form a neat, symmetrical dome. The most intense rising and sinking air often occurs around the edges of the high-pressure zone rather than directly beneath its center, especially when a nearby jet stream gives a dynamic boost to vertical air movement. Still, the term has become a useful shorthand for the public to understand why heat becomes so extreme and so persistent during certain events.
The 2021 Pacific Northwest Heat Dome
The most dramatic recent example struck the Pacific Northwest from roughly June 25 to July 2, 2021. Temperatures in parts of Washington state reached 120°F (48.9°C), shattering records in a region where many homes lack air conditioning. Critically, temperatures stayed dangerously high overnight, giving people’s bodies little chance to cool down.
The toll was severe. Across British Columbia, Oregon, and Washington, the combined excess death toll reached at least the high hundreds. In Washington alone, approximately 159 excess deaths occurred during the event and in the two weeks that followed, making it the state’s deadliest recorded weather event. Rapid attribution analysis concluded that human-caused climate change made an extreme heatwave of that magnitude at least 150 times more likely than it would have been in a pre-industrial climate.
Climate Change and Increasing Risk
Heat domes themselves are not new. High-pressure blocking patterns have always been part of atmospheric behavior. What is changing is the baseline temperature on which they build. Research published in Nature Communications found that hot extremes intensified noticeably after the 1990s, driven by accelerated background warming rather than by any clear increase in the blocking circulation patterns themselves. In other words, the atmospheric “lid” doesn’t need to get stronger; it just needs to sit on top of an already warmer atmosphere to produce more dangerous outcomes.
The probability of 2021-scale heat extremes is projected to keep rising due to background warming and a feedback loop between dry soil and the atmosphere. When the ground dries out during a heat dome, less energy goes into evaporating moisture and more goes into heating the air, which pushes temperatures even higher. Limiting global warming to 1.5°C instead of 2°C would avoid 53% of the projected increase in population exposure to extreme heat events of that severity. At 3°C of warming, the avoided exposure jumps to 89%, meaning a vastly larger number of people would face life-threatening heat.
Why Heat Domes Are Dangerous to Your Body
Your body cools itself primarily by sweating. When sweat evaporates from your skin, it carries heat away. But during a heat dome, both temperature and humidity can climb to levels where this cooling mechanism starts to fail. Scientists measure this using something called the wet-bulb temperature, which accounts for both heat and humidity. A wet-bulb temperature of 35°C (95°F) was long considered the theoretical upper limit of human survival, but lab experiments on young, healthy adults found that the actual threshold is considerably lower.
In humid conditions, subjects could no longer maintain a stable core body temperature once wet-bulb readings hit about 30.5°C to 31°C (roughly 87°F to 88°F). In hot, dry conditions, the limit dropped further, to around 25°C to 28°C wet-bulb (77°F to 82°F). These are values that already occur during severe heat domes, and the subjects tested were young and healthy. Older adults, people with chronic conditions, and those without access to cooling face danger at lower thresholds still.
Nighttime temperatures matter enormously. Under a heat dome, the sinking air and clear skies can keep overnight lows in the 80s°F or higher. Your body depends on cooler nights to recover from daytime heat stress. When that recovery window disappears, the cumulative strain on the heart and organs builds with each passing day. The World Health Organization considers indoor temperatures above 24°C (75°F) to carry potential health risk for sedentary people, a threshold easily exceeded in homes without air conditioning during a multi-day heat dome.
Staying Safe During a Heat Dome
The most important thing during a heat dome is access to cooling. If your home doesn’t have air conditioning, spending even a few hours per day in a cooled public space like a library, mall, or designated cooling center can significantly reduce the physiological strain on your body. Fans help when the air temperature is below your skin temperature (about 95°F), but above that point, they mainly blow hot air over you and can actually speed up dehydration.
Hydration matters, but so does timing. Avoiding exertion during peak afternoon hours, wearing light and loose clothing, and checking on neighbors who live alone, particularly older adults, are practical steps that save lives during these events. The deadliest feature of a heat dome is its duration: a single hot day is manageable, but a week of relentless heat with warm nights is when fatalities spike.