What caused the US heat wave?

First, a "heat dome," or area of high pressure. Sinking air from the Earth's atmosphere prevents air near the ground from rising. That sinking air operates like a cap, trapping warm ground air in place, according to the U.S. National Oceanic and Atmospheric Administration. Without rising air, there is also no rain, and nothing to stop hot air from becoming hotter.

That high pressure works in combination with a two-decade dry spell that has sucked moisture out of soil in much of the western United States. Usually, some of the sun's heat evaporates moisture in the soil, but according to the Associated Press scientists say the Western soil is now so dry that that energy is instead used to make the air even warmer.