In the summer of 2021, temperatures in Portland, Oregon hit 116°F — a city where few homes have air conditioning and the all-time record had previously been 107°F. Hundreds of people died. Across the border in Lytton, British Columbia, Canada recorded its hottest temperature ever at 121°F, then burned to the ground in a wildfire the next day. The culprit behind that catastrophic week was a single weather phenomenon: a heat dome.
Heat domes are not just hot weather. They are a specific atmospheric pattern that locks hot air in place, prevents the normal cooling that comes with night, and can push temperatures to extremes that a region has never experienced in recorded history.
A heat dome occurs when a strong area of high pressure parks itself over a region and refuses to move for days or weeks. High pressure systems push air downward from the upper atmosphere. As that air descends, it compresses and warms — the same physics that makes a bicycle pump feel hot when you use it. The result is a cap of hot, sinking air that sits over the surface like a lid on a pot.
The "dome" name comes from the shape of the high-pressure ridge when you look at it on a weather map. The upper atmosphere bulges upward in a dome shape over the affected area, and the sinking air beneath it suppresses cloud formation, prevents thunderstorms from developing, and blocks cooler air masses from moving in. Sunshine pours in unobstructed day after day while the air stagnates.
The primary driver is the jet stream — the fast-moving river of air in the upper atmosphere that steers weather systems around the globe. Under normal conditions, the jet stream follows a relatively flat, west-to-east path. But it can develop large north-south waves called Rossby waves, which cause it to meander dramatically.
When the jet stream buckles into a deep ridge — a pronounced northward bulge — it allows a mass of high pressure to build beneath it. The heat dome sits in the middle of that ridge. Meanwhile, the jet stream's path around the sides of the ridge actually blocks cooler air from breaking through and dislodging the system. The more amplified the ridge, the longer the heat dome holds.
Ocean temperatures also play a role. Research suggests that unusually warm ocean surface temperatures in the western Pacific can set off a chain of atmospheric waves that ultimately amplifies the jet stream ridge over land thousands of miles away — the same mechanism implicated in the 2021 Pacific Northwest event.
The core danger of a heat dome is persistence. A single hot afternoon is unpleasant; five consecutive days of record-breaking heat with no overnight relief is deadly. Here is why the combination is so much more lethal than the temperature alone suggests:
No nighttime cooling. Normally, temperatures drop significantly after sunset, giving bodies, buildings, and cities time to shed heat. Under a heat dome, nighttime lows remain dangerously high — often 80°F or above. People sleeping in un-air-conditioned homes accumulate heat stress night after night with no recovery window.
Urban heat amplification. Cities trap heat in asphalt, concrete, and buildings. The urban heat island effect can add 5–10°F to ambient temperatures, pushing areas that are already under a heat dome into extreme danger territory, especially overnight.
Infrastructure stress. Power grids strain under the simultaneous air conditioning load of millions of households. In the 2021 Pacific Northwest event, power outages in areas with no backup cooling turned homes into ovens. Roads buckle, rail lines warp, and cooling centers are overwhelmed.
Wildfire risk. The same dry, stagnant, hot conditions that bake people also desiccate vegetation rapidly. Within days, landscapes that had normal moisture levels become tinder. The Lytton wildfire in 2021 ignited the day after the record temperature was set.
| Event | Peak Temperature | Deaths (est.) |
|---|---|---|
| Europe, 2003 | 117°F (47°C) in Portugal | ~70,000 |
| Russia, 2010 | 111°F (44°C) in Moscow | ~55,000 |
| Europe, 2019 | 115°F (46°C) in France | ~2,500 |
| Pacific Northwest, 2021 | 121°F (49°C) in Lytton, BC | ~1,400 |
| UK / Western Europe, 2022 | 104°F (40°C) in the UK — first ever | ~3,200 |
What stands out across these events is that they consistently broke records not by a fraction of a degree but by margins of 5°F to 10°F or more — thresholds far outside the range of anything the infrastructure, population, or ecosystems of those regions had been built to handle.
A heat wave is the outcome — an extended period of unusually hot weather relative to normal for a region. A heat dome is a specific atmospheric mechanism that can cause one. Not every heat wave is produced by a heat dome, and not every heat dome produces a record-breaking heat wave. But the most extreme, prolonged heat waves — the kind that make international headlines — are almost always driven by a heat dome.
Think of it this way: a heat wave is the symptom, a heat dome is the diagnosis.
Most heat dome events last between 5 and 14 days before the jet stream pattern shifts enough to allow cooler air to break through. However, in rare cases — particularly when an unusually persistent blocking pattern sets up — they can persist for three to four weeks. The longer the event, the higher the cumulative death toll, because infrastructure fails, people exhaust their adaptive capacity, and emergency services are overwhelmed.
The end of a heat dome often comes with dramatic thunderstorms as the cooler air mass finally undercuts the high pressure. After days of suppressed convection, the atmosphere releases accumulated energy rapidly, sometimes with severe weather.
The short answer is yes, in terms of their impact. A heat dome that would have been survivable 50 years ago becomes more dangerous each year simply because baseline temperatures are higher — the dome is sitting on top of a warmer starting point. Research also suggests that Arctic warming is slowing and destabilising the jet stream, making it more prone to developing the deep, persistent ridges that produce heat domes.
The 2021 Pacific Northwest event was studied intensively by climate scientists, who concluded it would have been "virtually impossible" without human-driven climate change, and that events of that magnitude are now expected to occur roughly every 5 to 10 years rather than once per millennium.
Check the forecast early. Heat dome events are foreseeable 5–7 days in advance in modern weather models. If a multi-day extreme heat forecast appears, start preparing — not waiting until day three.
Find air conditioning. A few hours per day in a cooled space — a library, mall, or cooling centre — is enough to give your body recovery time even if your home has none.
Focus on overnight conditions. If your bedroom stays above 80°F at night, consider a ground-floor room, a fan blowing across ice, or an emergency cooling solution. Overnight recovery is more important than daytime comfort.
Check on neighbours. The 2003 European heat wave killed tens of thousands of elderly people who were alone. A single daily check-in can save a life.
Watch the heat index, not just the temperature. High humidity under a heat dome makes conditions even more dangerous than the thermometer suggests. Check your actual felt temperature before planning outdoor activity.
→ Check today's temperature and heat index for your city on ClearCast