What is a heat dome that's causing extreme heat and wildfires in Europe?

Heat domes are becoming a more regular weather phenomenon as entire regions deal with increasingly extreme heat and wildfires.

Villages in Turkey, Greece and Italy have been engulfed this July and August, and tourists evacuated by boat from beaches as temperatures reached 47C (116F).

In June, record temperatures hit North America, with more than 100 people dying in the northwestern US and Canada.

Both these extreme weather events were caused by heat domes.

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What is a heat dome?

It’s when an area of high pressure stays over a large part of a region for days, or even weeks.

Like a lid on a saucepan, it traps hot air underneath, and can cause heatwaves with temperatures well above the norm.

How does a heat dome form?

Hot air expands vertically into the atmosphere then high pressure from above means it has nowhere to escape and pushes that warm air down.

As the warm air sinks, it compresses and heats up, which then traps more heat underneath.

The ground then heats up and loses moisture which makes it heat up even more, and means it is ripe for fires to start.

The dome of high pressure also pushes the clouds around it, keeping the heat in even more.

Usually, winds can move the high pressure around but as the dome stretches high into the atmosphere, the high pressure system becomes very slow moving, almost stationary.

What has caused the European heat dome?

Met Office spokesman Stephen Dixon told Sky News: “The jet stream has dipped south across western Europe and extended into northeast Europe, allowing a ridge to develop across southeast Europe.

“Within the ridge, the air has become warmer day-on-day.”

Warm air from a Saharan dust cloud has also contributed to the warmer than usual temperatures

The high pressure from the jet stream ridge and the Saharan warm air has been stuck over southeast Europe for a while, maintaining temperatures 10C to 15C above average.

Are heat domes rare occurrences?

They are quite common in temperate zones but they are getting more intense and regular in areas that do not usually see such extreme heat.

Scientists from the US National Oceanic and Atmospheric Administration (NOAA) found the main trigger is a strong change in ocean temperatures during the preceding winter.

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For the US, this happens in the Pacific Ocean.

The NOAA scientists said it is like a swimming pool when the heater is turned on – “temperatures rise quickly in the areas surrounding the heater jets, while the rest of the pool takes longer to warm up”.

They said the western Pacific’s temperatures have risen over the past few decades compared with the eastern Pacific, “creating a strong temperature gradient – or pressure differences that drive wind – across the entire ocean in winter”.

The gradient causes more warm air through convection, which is heated by the ocean surface and rises over the western Pacific, decreasing convection over the central and eastern Pacific.

Prevailing winds move the hot air east, towards the US, and the jet stream traps the air, moving it towards land where it sinks to cause heatwaves.

In Europe, the water temperatures are high, especially across the Baltic region where they are more than 6C above normal.

The Atlantic Ocean around the UK and Ireland was about 2-4C above the norm for the end of July.

But it is the Mediterranean, which is warmer than other European seas anyway, that is the most concerning, with sea temperatures nearly 3C above the long-term average.

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