A heat dome is causing excessive heat in some regions of the country.

Parts of California, Nevada, Arizona, Utah, and Texas are under an excessive heat warning this week. Forecasters say the heat could spread east later in the week.

As of Tuesday afternoon, more than 36 million Americans were under heat alerts, according to the National Weather Service.

Heat domes occur when the atmosphere traps hot air from the ocean.

“Extreme heat is probably the most deadly weather disaster that we have, ” Climate Central chief program officer Andrew Pershing said.

About 1,220 people are killed by extreme heat every year in the U.S., according to the CDC.

During a heat wave, the National Weather Service recommends wearing light-colored and loose-fitting clothing, drinking plenty of water, eating light, and minimizing direct exposure to the sun. Reducing or eliminating strenuous activity outside is also important.

This equates to about $58 more per family for the period of June through September, according to projections. The total cost of cooling for that period has risen from about $476 10 years ago, to a predicted $719 this summer.

The heat also creates an increased risk for wildfires, which California has already started seeing with the more than 14,000-acre Corral Fire east of San Francisco.

While summer has not officially started yet, this is the first triple-digit heat wave for many areas in the Southwest.