What a heat dome is – and why it isn't behind the UK's soaring temperatures

Parts of the UK are experiencing a heatwave at the start of September, while a ‘heat dome’ has been behind sweltering temperatures in Europe

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

September has got off to a hot start and parts of the UK have already hit the heatwave criteria for the first time since June.

Following one of the wettest Julys on record, August was more of a mixed affair, with cooler temperatures, rainfall and some warmer weather. However, temperatures have spiked again – and the hottest day of the year is expected on Saturday.

And it's not just the UK. Temperatures reached 39C in some parts of France on Monday, while the average temperature for the country was 25.1C on the same day – making it the hottest ever September day recorded in the country.

In recent weeks, swathes of southern Europe across France, Greece, Italy, Turkey and Portugal have experienced severe droughts that have fuelled wildfires, prompted health warnings and resulted in more than 60,000 heat-related deaths. Areas of Turkey, Bulgaria, Spain and Greece have since seen soaring temperatures replaced by sweeping floods this week.

However, while the current UK weather is down to high pressure in continental Europe pulling warmer air over the UK, the searing temperatures in Europe were caused by something known as a ‘heat dome’.

London, UK. 5th September, 2023. UK Weather: Heatwave in the city. Locals enjoy a warm evening sunset in Blackheath Park. The UK is set to be hotter than Ibiza, Ayia Napa and Athens this week, forecasters say, while other parts of Europe are experiencing a weather phenomenon called a
A runner makes his way through Blackheath Park in London as temperatures in the UK approach 30C. (PA) (Guy Corbishley)

What is a heat dome?

Met Office spokesman Stephen Dixon told Yahoo News UK that a heat dome effectively refers to “a static area of high pressure that allows temperatures to build day-on-day”, adding that this “is not what the UK is experiencing at the moment”.

Essentially, 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.

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’.

“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.”

Climate change: read more

A man walks on the Capitole place (the main square in toulouse) under a shade device. France is suffocating under a new heatwave. Temperature records are and will be broken during this heatwave especially in the south. France is 8 to 15?C above normal temperatures for August. This heatwave is due to a heat dome and is qualified as 'extreme for the intensity, the duration and for being in late summer'. More than 15 departments such as Haute-Garonne (Toulouse) are placed on red alert on heat. Red alert on heat is announced when night temperatures are above 26?C and day temperatures are above 40?C.  Water restrictions are in place in all France, someplaces haven't drinking water and must be resupply with trucks. Toulouse. August 22th 2023. (Photo by Alain Pitton/NurPhoto via Getty Images)
Water restrictions were put in place in France as the country saw temperatures rise due to a heat dome. (Getty) (NurPhoto via Getty Images)

Are hot Septembers in the UK unusual?

The cause of the UK's current blistering weather has been put down to an ”omega block”.

According to Dr Monica Ionita, climate scientist at the Alfred Wegener Institute, this weather system is behind the spike in UK temperatures – and the flooding in southern Europe.

She wrote on X, formerly known as Twitter: ”The current weather situation in Europe is a textbook omega block. While central Europe will ’enjoy’ a heatwave, Greece will face a #medicane with disastrous damages due to heavy rain and floods fueled by an extremely hot MediterraneanSea.”

US meteorologist Jeff Berardelli posted this video highlighting how the impact has unfolded:

While September temperatures approaching the 30s aren’t unheard of in the UK, the longevity of the current heat event is “unusual”, Met Office spokesman Stephen Dixon told Yahoo News UK.

He explained: “The record highest temperature for the UK in September is 35.6C and was set as far back as 2 September 1906.

“However, the heat we’re seeing at the moment is notable for its longevity.

“We had three consecutive days of temperatures in excess of 30C in September on a handful of occasions in our climate records, though this week we look likely to see a few more days than that in terms of a specific temperature being above 30C for consecutive days.“

Dixon said that in a changing climate, heat events “are likely to become more intense, more frequent and longer-lasting”.

However, he added that the “natural variability” – that was seen in the UK with a largely unsettled July and August – will continue.

A woman walks at a park during a sunny day in London, Wednesday, Aug. 23, 2023. (AP Photo/Kin Cheung)
Hot weather in September is not unusual – but its longevity is notable, forecasters said. (AP) (ASSOCIATED PRESS)

A climate problem for today, and tomorrow

The World Meteorological Association has 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 recently.

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.

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.”