Source: CNN

A heat dome occurs when a ridge of high pressure builds over an area and doesn't move for up to a week or more. High pressure results in fair weather with lots of sunshine and very few clouds. It also indicates sinking air, and when air sinks, it warms - causing temperatures to rise. The "dome" is created because the air can't escape. Then, temperatures keep warming, often to uncomfortable or even dangerous levels.

Most heat records are set within a heat dome. And the climate crisis is expected to make them more frequent - and even hotter.

In 1995, more than 700 people died in the Chicago metro area as a heat dome settled over the Midwest. Temperatures topped 100 degrees but felt closer to 125 because of the heat index. Many died because overnight temperatures stayed warm, so bodies couldn't recover from daytime heat.