As temperatures continue to reach exceptional levels across parts of the world, one term appears again and again in news reports: heat dome.
It is often used to describe some of the most intense and long-lasting heatwaves, particularly those that shatter long-standing temperature records.
But what exactly is a heat dome, how does it develop, and are these events becoming more severe in a warming climate?
The heatwave that brought the term into the spotlight
The phrase gained widespread attention during the extraordinary western Canada heatwave of 2021. Forecasts initially suggested temperatures could rise well beyond previous national records, but even experienced meteorologists were surprised by the scale of what eventually unfolded.
The village of Lytton recorded 49.6°C, breaking Canada's national temperature record by an unprecedented margin. Remarkably, the record was broken repeatedly over consecutive days, highlighting just how exceptional the event was.
As the heatwave dominated headlines around the world, one phrase became firmly associated with the event: heat dome.
What is a heat dome?
A heat dome develops when a large area of high pressure becomes established over a region and remains there for an extended period. Under high pressure, air sinks through the atmosphere. As it sinks, it becomes compressed and warms. At the same time, cloud formation is suppressed, allowing strong sunshine to reach the ground.
The longer this pattern persists, the greater the heating effect. Clear skies encourage daytime temperatures to rise, while soils gradually dry out. As ground moisture decreases, less solar energy is used for evaporation and more becomes available to directly heat the land surface and the air above it.
This creates a feedback loop where dry ground promotes additional warming, which in turn encourages further drying. Because warm air occupies a deeper layer of the atmosphere than cooler air, the resulting pattern can resemble a dome-shaped area of heat that becomes difficult to dislodge.
At the Met Office, the term "heat dome" is used less frequently, with meteorologists often referring instead to persistent summer blocking highs. However, the underlying meteorology is largely the same.
READ MORE: UK heatwaves: How the records tell a changing story
Why the UK experiences extreme heat
While hot air arriving from the continent often receives much of the attention during UK heatwaves, the reality is more complex.
Meteorologists identify several key ingredients that contribute to the UK's most extreme heat events. One of the most important is the presence of a strong blocking high pressure system, which encourages sinking air and allows heat to build over several days.
Another factor is the transport of warm air from continental Europe, particularly from Iberia and France. This process can add further warmth to an already favourable environment for extreme temperatures.
Dry ground conditions can also make a significant difference. When soils contain little moisture, less of the sun's energy is used to evaporate water. Instead, more energy goes directly into heating the surface and the air above it, enabling temperatures to rise more efficiently.
It is often the combination of these ingredients which produces the UK's most notable heatwaves.
Why are records being broken by larger margins?
One striking feature of recent heatwaves is not simply that records are being broken, but by how much they are being exceeded.
In previous decades, temperatures often built gradually during a warm spell, perhaps reaching around 30°C after a week of settled weather. Today, temperatures can climb far more rapidly and reach much higher values.
Meteorologists point to the influence of a warming climate. Even when familiar weather patterns occur, they are now developing within a warmer background environment. This means that blocking highs that may once have produced a notably warm spell can now generate considerably more extreme temperatures.
As a result, temperature records are increasingly being exceeded by larger margins than would once have been expected.
READ MORE: Warmest UK summer on record “increasingly likely” as temperatures stay well above average
Are heat domes becoming more common?
This is one of the key scientific questions currently being investigated.
Research suggests there is still uncertainty over whether climate change will lead to more frequent blocking highs or significantly alter the behaviour of the jet stream. Scientists continue to study these complex atmospheric processes, and there is not yet a definitive answer.
However, studies indicate that even if blocking highs become less frequent in some regions, the heat associated with them is likely to become more intense. One reason for this is the increasing interaction between drier soils and the atmosphere, which can amplify heat during these events.
The broader warming of the climate also plays a major role. When temperatures start from a higher baseline, extreme heat becomes more likely and more severe.
Why understanding extreme heat matters
The impacts of extreme heat extend far beyond simply uncomfortable weather. Events such as the 2021 Canadian heatwave demonstrated how prolonged, intense heat can affect health, infrastructure and daily life, particularly when temperatures remain elevated overnight.
As heatwaves become more intense, forecasting is only one part of the challenge. Effective warning systems and heat action plans help individuals, communities and organisations prepare for dangerous conditions and reduce risks.
The bigger picture
So, are heat domes getting worse? The answer is nuanced. Scientists are still investigating how climate change may influence large-scale weather patterns such as blocking highs. What is clear, however, is that when these patterns occur, they are now operating in a warmer climate and often over drier ground.
That combination means the heat they produce has the potential to be more intense than in the past. In other words, even if the weather patterns themselves do not change significantly, their impacts are becoming increasingly severe.
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