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A record-setting heat wave in the US has affected nearly a third of the population as of Monday. The wave scorched the American Southwest and is now headed for the East Coast. Last week, temperatures reached up to 123°F in California’s Death Valley and 100°F in Las Vegas. Through Thursday, the heat wave is expected to cause forest fires and a greater risk of scattered thunderstorms and tornadoes, the National Weather Service warns.

A heat wave occurs when unusually hot weather lasts for at least two days. In this case, high atmospheric pressure has trapped hot air that moved inland from the Pacific Ocean. Like a closed lid, the upper layer of air has pushed the hot air back down, creating a “heat dome” that prevents the ocean air from escaping. The heat dome stops clouds from forming, making it less likely for rain or cloud cover to block the sun’s sweltering heat. 

“It’s a typical type of setup when you have a heat wave,” Robert Oravec, a meteorologist at the National Weather Service, told The New York Times. “When these types of patterns develop, you tend to have subsiding air, and the subsiding air warms the atmosphere. And you can get really warm temperatures.” 

Amid global warming, severe heat waves will soon become a summertime norm—and they will exceed the waves experienced in the past two decades. “Climate change is increasing the frequency and intensity of heat waves around the world, tilting the scale in the direction of warmer temperatures,” Brandon Miller, a meteorologist and climate expert, told CNN.

[Related: How to stay cool if you lose power during a heatwave]

As the heat wave moves east, more than 100 million Americans, such as those living in Chicago and Memphis, have received heat warnings or heat advisories. This is because temperatures are expected to rise from 10°F to 30°F above normal.

“Even cities as far east as Charlotte and Raleigh, North Carolina, will be flirting with record high temperatures and triple-digit readings this week,” Dan Pydynowski, a meteorologist for Accuweather, told USA Today.

Extreme heat can cause heat-related illnesses and is responsible for over 600 deaths every year. The National Weather Service in Phoenix, where temperatures are up to 115°F degrees, wrote on Twitter that “heat is the #1 weather-related killer in the U.S. and AZ, so take the proper actions to protect yourself from the heat.” 

The Centers for Disease Control and Prevention advises staying indoors as much as possible during a heat wave and drinking lots of water, even when you’re not feeling thirsty. If you must go outside, apply sunscreen and wear loose, light-colored clothing. Pace yourself when in the sun, and have a friend or family member check up on you to ensure you’re okay.

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