The southwestern United States is experiencing a heat wave. Several heat records have been broken in Las Vegas and cities near San Francisco.

In Ukiah, north of San Francisco, a record temperature of 47 degrees Celsius was recorded. It’s even warmer in Death Valley, California, one of the hottest places on Earth. In 1913, a temperature of 56.7 degrees was measured there. That’s still a world record, though some question the accuracy of the measurement. Today, the temperature is expected to be about 54 degrees in Death Valley.

The weather service in Phoenix, where temperatures this week reached 105.5 degrees, said the heat was “concerning.” Several people had to go to the hospital because of the heat. Local police said a 10-year-old boy died Tuesday after a “heat-related accident” while out for a walk.

Several large wildfires have broken out in California, fueled by heat and wind. Tens of thousands of people have had to be evacuated due to the fires.

Meteorologists expect the heat to get even more intense. After the weekend, temperatures could rise to 47 or 48 degrees. The extreme heat looks set to last for about a week, especially in the western part of the United States, and spread to the Northwest.

The National Weather Service (NWS) has issued a “dangerous heat” warning. The warning applies to more than 130 million Americans. That’s about 40 percent of the population of 335 million.

Because of the high humidity in the area, it’s difficult for residents to lose body heat. The weather service expects nights to provide some cooling as well. The NWS advises people to drink plenty of water, stay out of the sun and seek cool places.