What is a ‘heat dome’? Understanding the weather pattern causing record temperatures this summer

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Watch: Heat dome - what is the extreme weather pattern causing record temperatures and wildfires?

"Heat domes" have been in the news after Greece experienced heat of up to 47C this month and record temperatures swept North America earlier this year.

Both the American and European heatwaves have been attributed to the phenomenon, which sees heat getting trapped in an area for days or even weeks without being moved by wind.

In a heat dome, the heat is trapped by a "lid" of high-pressure air.

With the heat unable to escape an area, more warm air heats up and rises, becoming compressed and trapping even more heat.

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The dome stretches high into the atmosphere and becomes "locked" over an area.

It also dries the ground and can create the perfect conditions for fires.

According to the US NOAA weather service, “vast areas of sweltering heat get trapped under the high-pressure ‘dome’.

EVIA, GREECE - AUGUST 10: Firefighters try to extinguish fire in the island of Evia, Greece on August 10, 2021. Wildfire has destroyed hundreds of homes, thousands of hectares of forests and forced hundreds of people to flee in eight days on the Greek island of Evia. (Photo by Ayhan Mehmet/Anadolu Agency via Getty Images)
Firefighters try to extinguish fire on the island of Evia, Greece, on 10 August, 2021.(Ayhan Mehmet/Anadolu Agency via Getty Images)

‘High-pressure circulation in the atmosphere acts like a dome or cap, trapping heat at the surface and favouring the formation of a heat wave.”

When a record-breaking heatwave roasted Canada earlier this summer, Armel Castellan, a meteorologist with Environment and Climate Change Canada, said: “We have experienced a ridge with low pressure sandwiched on either side.

"It’s really hard to move it. The jet stream isn’t moving it along. In that pattern we have essentially a heat dome.

"A pattern that is sticking to its guns where pollutants and heat keep adding to each other. It is compounding."

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This month’s heatwave in Greece saw fires blocking out the sun and the hottest weather in 30 years.

The World Meteorological Association said climate change meant heatwaves were becoming both more frequent and more intense.

Extreme weather events that previously would have happened every 50 years could soon happen every four if temperatures rise 2C above pre-industrial levels, a report from the UN's Intergovernmental Panel on Climate Change warned this week.

The report is the first to quantify the likelihood of extreme events across a wide variety of scenarios.

Dr. Robert Rohde, lead scientist of Berkeley Earth, said: “What were once-in-50-year heat extremes are now occurring every 10 years.

“By a rise of 2C, those same extremes will occur every 3.5 years.”

The report found that (for example) once-in-a-decade heavy rain events are already 1.3 times more likely and 6.7% wetter, compared with the 50 years leading up to 1900 when human-driven warning began to occur.

Droughts that previously happened once a decade now happen every five or six years.

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Xuebin Zhang, a climatologist with Environment Canada in Toronto, warned that as the world warms, such extreme weather events will not just become more frequent, they will become more severe.

Zhang said that the world should also expect more compound events, such as heatwaves and long-term droughts occurring simultaneously.

“We are not going to be hit just by one thing," Zhang said, "we are going to be hit by multiple things at the same time.”

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