Conceptual illustration of a heat dome over Britain. A heat dome is a weather phenomenon where hot air is trapped over a certain area, causing extreme heat. They occur when a strong high-pressure system in the upper levels of the atmosphere becomes stationary over a region. This causes warm air to be confined beneath the stalled high pressure, while sunshine further intensifies the heat. The effect is akin to placing a lid on a pot, trapping the hot air and preventing it from escaping.