What is a 'heat dome' that may affect FIFA WC players & fans?

Extreme heat is expected to affect fans and players this week at the FIFA World Cup 2026 as an imposing 'heat dome' settles over parts of the US and Canada. A 'heat dome' is a large area of high pressure that traps heat and humidity. It could push heat indices to 40.5℃-46.1℃, according to the US National Weather Service.