A historic heat dome is sitting over Europe, and it's been breaking records in several countries for days.

In France, Tuesday was officially the hottest day ever recorded in the country’s history. Météo-France, the weather service in France, reported on social media that a thermometer in Pissos (Landes) reached 44.3 degrees Celsius, or about 111 degrees Fahrenheit. "Many cities experienced unprecedented values, regardless of the month, including 42.1 °C in Bordeaux," the agency said.

Widespread temperatures above 40 degrees Celsius (104 degrees Fahrenheit) forced the Eiffel Tower and the Louvre to cut visiting hours. French authorities say 40 people have now drowned in the past week alone, as desperate residents jump into rivers and lakes to cool off.

Across the Channel, "London isn’t just calling. It’s cooking," says the UN secretary general, António Guterres. The Met Office has issued a rare extreme heat warning, with London on track to hit 40 degrees Celsius by Thursday. That would be only the second time that’s ever happened in British history. The first was back in 2022.

This heat dome is sweeping across the continent 50 years to the day after a historic 1976 heat wave killed 250 people and caused food prices in Britain to rise by 12%.

So what exactly is a heat dome? It’s a stubborn area of high pressure that parks itself over a region and traps hot air underneath. The longer it sits, the hotter the ground gets, and the worse it gets day after day.

Heat domes can form anywhere conditions line up — including right here in the U.S. A strong heat dome fueled the deadly Pacific Northwest heat wave in 2021 that caused over 250 deaths in the U.S. and more than 400 in Canada.

Scientists say climate change is making these events more frequent, more intense, and more likely to shatter records. Extreme heat is already the deadliest weather hazard, killing more people globally than floods, hurricanes, and wildfires combined.