A heat dome pushed daytime readings past 100F (38C) across much of the western United States in mid-July 2026. On one Sunday it set all-time heat records in two states at once. Billings, Montana, hit a sweltering 111F (43.9C), its hottest day on record, and Salt Lake City, Utah, reached an all-time high of 109F the same day.
What a heat dome actually is
A heat dome forms when a large area of high pressure settles over a region and refuses to move on for days or even weeks. Air trying to rise gets pushed back down, compresses, and warms further, while the same high pressure keeps clouds from forming, so unbroken sunshine keeps beating on the ground below. AccuWeather meteorologist Brandon Buckingham describes the effect as a dome that acts “like a lid”. It is not a formal scientific term, but it has become a popular way to describe the pattern in recent years.
During the 2021 Pacific Northwest heat dome, the village of Lytton, in British Columbia, set record-breaking temperatures on three consecutive days, peaking at 49.6C. Seattle and Portland set all-time highs of their own that week, and hospitals filled with people suffering heat-related illness as governments relaxed social-distancing rules so residents without air conditioning could shelter in emergency cooling centres.
What it is doing right now
This year’s heat has arrived alongside a punishing wildfire season across the western US. Three firefighters died in late June battling a blaze along the Colorado-Utah border. A helicopter pilot responding to a separate Colorado fire also died this week, in a crash whose cause remains unclear, the Guardian reports. Smoke from more than 100 wildfires burning in Canada gave Toronto the worst air quality of any city in the world this week, with winds carrying that pollution south into the US.
Cities make the heat worse for the people living in them. Concrete and asphalt soak up the sun’s energy by day and give it back after dark. That can make a heat island up to 11C (20F) hotter than the open countryside nearby, and roughly 41 million Americans live in areas with extreme urban heat island temperatures, according to the BBC. Los Angeles has required cool roofs on new homes since 2013, and Phoenix has approved more than $1.4m to plant up to 1,800 trees for shaded corridors through its hottest neighbourhoods.
Why it matters beyond one hot week
Climate Central found that climate change made a 2025 heat dome at least five times more likely. Shel Winkley, the group’s weather and climate engagement specialist, compares the mechanism to a crowd packed too tightly together: sinking, compressing air molecules bump into each other and heat up, he says, “like a mosh pit”.
The background climate keeps shifting upward, so a blocking pattern that once produced a warm spell now produces a record. The Intergovernmental Panel on Climate Change’s 2023 assessment found land-based heat extremes hitting harder and more often than they did in the middle of the last century. Steffen Lehmann, who runs the University of Nevada’s Urban Futures Lab, says the fixes are not a mystery: cities know how to cool themselves down, and most have not done it. The trouble, he says, is “too much talking and not enough action”.
