How a heat dome is formed and why experts blame one for Europe's baking temperatures

Europe is sizzling under an early heat wave this week, with millions of people experiencing extremely high temperatures, and experts say a phenomenon known as a heat dome is to blame.

Here's what to know.

What is a heat dome?

Heat domes are essentially high pressure systems that remain stationary for a few days, trapping dangerous heat and humidity, said Mireia Ginesta, a research associate at the Climate Litigation Lab at the Smith School of Enterprise and the Environment.

Heat domes result from a northward bulge in the jet stream — a river of fast-moving wind at high elevations — that create the weather we experience.

"High pressure system means that the air is sinking, and as the air goes down to lower altitudes, it becomes compressed," Ginesta said. "So the pressure increases and the temperature also increases."

How does a heat dome play a role in heat waves?

Those "bulges" are what set up the conditions that lead to a heat wave, said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center.

"The heat dome is really what the jet stream is doing," Francis said. "The heat wave is what we feel at the surface."

What is happening in Europe this week?

Millions of people across the continent have been experiencing exceptionally high temperatures as an early summer heat wave sears France, Italy, Spain and the United Kingdom.

"In Europe, they're just not used to this," Francis said. "It's really just in the last decade or two where these sorts of really brutal heat waves have been happening and killing a lot of people because they don't have the means to stay cool."

France, which has been the most affected so far, doesn't have widespread air conditioning, and about half the country has been placed under a red heat wave alert by the national weather service. The nation has also reported around 40 fatalities because of drowning, as people sought cooling relief.

Those conditions are expected to last for several days, with temperatures as high as 104 degrees Fahrenheit (40 degrees Celsius).

"We are going to see the June temperature records not just broken, but completely annihilated," said Liz Bentley, chief executive at the Royal Meteorological Society and a professor of meteorology at the University of Reading.

How is climate change influencing these phenomena?

Climate change is making the conditions for heat domes happen more often, experts say. And more and more nations around the globe are being impacted.

"We're warming the globe and that means we're shifting the range of temperatures that any given place experiences," Francis said. "And as you shift that range of temperature, you're making the extreme temperatures much more likely."