Guide

What is a heat dome?

A heat dome is a large, persistent area of high pressure that traps hot air beneath it. Air sinking inside the high compresses and warms, the sinking motion suppresses cloud, and the pattern is slow to move — so heat accumulates over days instead of passing through in an afternoon.

The mechanism, in three steps

A large high-pressure system parks over a region. Usually because the jet stream has buckled into a slow-moving, high-amplitude pattern that leaves the high stuck in place.
Air sinks inside the high, and sinking air warms. Compression heating, the same physics that warms a bicycle pump. This is why a heat dome can produce temperatures far above what the sun alone would deliver.
Sinking air suppresses cloud, so the sun does the rest. Clear skies day after day, ground heating with no relief, and nothing moving through to break the pattern.

The name is a fair description: a lid of warm sinking air over a region, with the heat unable to escape upwards and nothing arriving from the side to displace it.

Why it is worse than the peak temperature suggests

Three compounding effects, and only the first is the one people talk about.

The ground dries out. Early in the event, soil moisture evaporates and that evaporation does real cooling. Once the soil is dry, that cooling stops and the same sunlight goes entirely into heating the air. The last days of a long event heat faster than the first.

Nights stop recovering. Days of stored heat in soil, roads and buildings, with no cloud and little wind, means the overnight minimum climbs through the event. That is the part that harms people: the human body handles a hot afternoon far better when the night that follows is cool, and mortality studies consistently find warm nights doing the damage.

Humidity can rise even as the temperature plateaus. Counter-intuitive, given that sinking air is dry, but soil and vegetation give up moisture into a shallow boundary layer. The result is a wet bulb temperature that keeps rising after the thermometer has stopped, which is the number that actually limits what the body can do.

What to watch, rather than the headline number

During a multi-day event, these tell you more than the afternoon maximum.
Watch	Why
Overnight minimum	Whether the body gets a recovery window at all. A night staying above ~25°C is the warning sign.
Wet bulb	Whether sweating still works. Can keep climbing after the temperature plateaus.
Day number	Cumulative strain and dried-out ground mean day five is not day one, at the same temperature.
Heat index band	The published categories, which is what advisories are written on. Calculate it.

What to do about one

Nothing exotic, but the multi-day shape changes the emphasis:

Plan for the whole event, not the afternoon. Cooling the sleeping space matters more on night three than the shade did on day one.
Check on people repeatedly. Older relatives and neighbours living alone, especially without air conditioning — and again later in the event, not only at the start. Heat risk in older adults covers why their line is lower.
Use the cool window while it exists. Early morning is the only genuinely cooler part of the day, and it shrinks as the event goes on. The hottest part of the day has the timing.
Take the alerts seriously even if the number looks survivable, because local thresholds account for what the local population is adapted to. Advisory, warning or watch explains which is which.

Common questions

What is the difference between a heat dome and a heatwave?

A heatwave is the outcome — a run of unusually hot days. A heat dome is one of the atmospheric patterns that produces one, specifically a persistent high-pressure system with sinking air. Not every heatwave involves a heat dome, but the longest and most severe ones usually do.

How long does a heat dome last?

Typically several days to a couple of weeks. Their persistence is the defining feature: the blocking pattern that sets one up also stops it from moving, so the same region bakes day after day while the ground dries out and stops being able to cool itself by evaporation.

Why does a heat dome make nights hot too?

Because the ground has been storing heat for days and there is little cloud or wind to move it, so night cooling is weak. Buildings, roads and soil re-radiate through the night. That matters more than the afternoon peak: without a cool night the body gets no recovery period, and it is multi-day events with warm nights that show up in mortality figures.

Does a heat dome increase humidity?

Often, in a way that surprises people. Sinking air is dry, so the first days can be very dry. But as soil moisture is drawn out by days of extreme heat and vegetation transpires, the lower atmosphere can become more humid — which raises the wet bulb even as the temperature plateaus, making conditions harder on the body.

Are heat domes becoming more common?

Assessments of recent events, including the 2021 Pacific Northwest event, have concluded that the temperatures reached were made substantially more likely by a warmer baseline climate. Whether the blocking patterns themselves are becoming more frequent is a more open research question than the temperature attribution.

Sources

Published 30 July 2026