New Delhi, June 25 -- 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 creates the weather we experience.

"High-pressure system means that the air is sinking, and as the air goes down to lower altit...