PHOENIX — A severe heat wave affecting 40 million Americans has seen temperatures over one hundred degrees Fahrenheit beat records in Wyoming, Utah, Arizona and Southern California, according to NBC News. It has two main causes, according to the Associated Press.

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.

As a consequence of the extreme heat at least 14 new wildfires broke out this week in Montana and Wyoming alone. Firefighters also fought fires in Arizona and New Mexico, with U.S. Department of Agriculture meteorologist Gina Palma saying these were "certainly conditions that we would not normally see in June."

Power networks across the country have also been strained due to increased use of air conditioning, according to Reuters. Operators in California asked homeowners across the state to conserve energy in the late afternoon and evening when demand surges.

In graphs published on its website, the U.S. Environmental Protection Agency showed that heat waves like this are almost three times as frequent as they were in the 1960s, increasing steadily for over 60 years. Furthermore, the duration of these heat waves is now almost a full day longer.