Heat Waves: How They Form and What Fuels Them

A lid that never lifts

Intense heat on its own is not enough to make a heat wave. Something has to hold it in place over a region long enough for it to build up, and in the vast majority of documented cases that something is a blocking high. In the mid-latitudes, high- and low-pressure systems normally travel from west to east, carrying heat away before it can build up. When an area of high pressure instead stops moving and stalls over a region for days, or in some cases weeks, cooler Atlantic or oceanic air can no longer get in: the heat stays put underneath, like under a lid nobody lifts.

Practical example: the British summer of 1976 formed exactly this way. A blocking high-pressure pattern stalled over the United Kingdom and trapped hot, dry air for weeks, keeping out the Atlantic weather systems that usually bring rain and relief. The main heat wave began on 23 June 1976, with temperatures consistently above 30°C across much of England, peaking at 35.9°C in Cheltenham on 3 July.

Why sinking air warms up on its own

Inside a blocking high a second, less intuitive process is also at work: air at the centre of the system tends to sink, a motion meteorologists call subsidence. As it descends, the air moves into a zone of higher pressure and compresses; compressing it warms up, much like the barrel of a bicycle pump heats up while you use it. This descending warm air also suppresses cloud formation, so the sky stays clear and the sun beats down unfiltered on the ground below.

When this effect spreads across a wide enough area, meteorologists sometimes call it a “heat dome”: a dome-shaped region in which hot air is trapped near the surface by stable atmospheric conditions. That is what happened in June 2021 over the Pacific Northwest, spanning Canada and the United States, when an area of high pressure stalled and produced extreme temperatures that lasted about a week and beat local records — some up to 70 years old — by more than 6°C in several places.

One lesser-known detail of that event: not all of the anomalous heat had formed on the spot. A substantial share had built up upwind of the block, over the Pacific Ocean, through so-called latent heating — the heat released when water vapour condenses into clouds and weather systems — which was then carried inland along with the moving air mass. The mechanism behind the water cycle — evaporation and condensation — is not only about rain: it can also feed a heat wave hundreds of kilometres from where the vapour first formed.

What adds fuel to the fire

A blocking high and subsidence explain most of a heat wave on their own, but they rarely act alone. Among the factors that can amplify its intensity:

• Dry soil. If drought precedes the arrival of the block, the ground already has little water left to give back to the air through evaporation: less evaporation means less natural cooling, so more heat builds up at the surface, which in turn dries the soil out further. It is a feedback loop that keeps feeding itself until the atmospheric pattern changes.
• The urban heat island. Asphalt, concrete, and buildings absorb more solar energy during the day than vegetation or open land, and release it at night: cities therefore tend to stay warmer than the surrounding countryside, even under the same heat wave.
• Land-use changes. Deforestation and intensive agriculture can alter the surface energy balance and moisture, adding to the effects of the blocking high.

Practical example: in the United Kingdom in 1976, the soil-atmosphere feedback played a direct role. The drought that preceded the heat wave had begun in May 1975 and continued until August 1976: by the time the blocking high arrived, the soil was already depleted of water, which helped intensify the heat. Documented impacts included water restrictions, failed harvests, and wildfires.

Two heat waves compared

The mechanisms behind the British summer of 1976 and the North American heat dome of 2021 look similar at their core, but the numbers tell very different stories:

The UK’s all-time temperature record, it should be said, does not belong to 1976: it was only broken in 2022, with 40.3°C measured at Coningsby. These are two different milestones — the hottest summer on average and the single highest temperature ever recorded — that often get mixed up.

Where a changing climate fits in

A high-pressure system can stall independently of global warming: it is an atmospheric phenomenon observed for decades, as the 1976 case shows. One of the factors climate change appears to add, according to Copernicus’s analysis, is a higher starting baseline for temperatures: if the average climate is already warmer, the same atmospheric patterns as always more easily produce extreme values, with heat waves that are more frequent, more intense, and longer.

For the 2021 event, studies gathered by Carbon Brief estimated that human-caused warming contributed around 1°C to the extreme temperatures recorded, by raising average summertime starting temperatures. The rest of the anomaly, and its statistical rarity — estimated at somewhere between 1 event every 200 years and 1 every 100,000, depending on the method — comes down to the local atmospheric mechanisms described above: blocking, subsidence, and heat carried in from upwind. It is a contribution that adds to that of the greenhouse effect, the underlying mechanism raising the planet’s average temperatures.

The public health impact of these events is far from negligible: the British Columbia Coroners Service attributed 619 deaths in the province to the 2021 heat wave, with a broader preliminary estimate putting the regional toll at least at 868 deaths; the heat wave also likely killed more than a million mussels along a single 100-metre stretch of coastline. In Europe, Copernicus estimates 61,700 heat-related deaths in 2022, 47,700 in 2023, and 62,800 in 2024, with heat-related deaths rising in 94% of monitored regions between 2000 and 2020.