More than 37 million Americans are under heat advisories across the U.S., according to the National Weather Service. Here's the hardest hit areas.

As the official start of summer arrives, the United States is being hit by a dangerous heat wave, making this the first extreme weather event of the season. A heat dome, a weather pattern in which warm air is trapped over a large region,

Extreme humidity and heat will affect around 265 million people across the country this week. Maps show where and when to expect the worst.