The National Weather Service declares a heat wave when there are five or more consecutive days of 90-degree weather or three or more consecutive days of 95-degree temperatures.
The service described a "heat dome" sitting over much of the nation, caused by a huge area of high pressure that traps and compresses hot, humid air from the Gulf of Mexico.
Already, almost half the U.S. population has been affected, and 22 deaths have reportedly been attributed to the heat wave. Across the United States, about 141 million people were under heat advisories and warnings.
According to the ABC:
In Minnesota, the heat index hit 134 degrees Tuesday. In Iowa, blistering heat buckled highways. And in South Carolina, residents prepared for a second major heat wave as the National Weather Service issued a heat advisory today.
The heat was also putting pressure on the nation's power grids:
Chicago had a heat index of 108 degrees on Wednesday afternoon, The Weather Channel reported. The heat index measures how hot it feels when humidity is added to the actual air temperature.
Many Chicago-area residents were dealing with power outages Wednesday, meaning no air-conditioning to help with the oppressive heat, ABC reports.
Some major U.S. cities prepared to keep public swimming pools open longer to help residents beat the heat.
And the Detroit Zoo planned to let visitors into air-conditioned attractions usually requiring an additional admission fee for free through Sunday, while the animals were to receive icy treats, cool showers and watermelon, The Detroit News reports,
Temperatures in the East felt like between 100 and 110 degrees on Wednesday, the National Weather Service said, and were set to get hotter on Thursday and Friday.
"After several days of deadly heat and humidity as many as 22 people have died," the National Weather Service said Wednesday, CNN reports.
The National Weather Service issued safety tips for adults looking to keep cool.
Slow down. Try to reduce or cancel any strenuous activities, or reschedule them for the coolest part of the day.
Wear light-weight, light-colored clothing to reflect sunlight and heat.
Eat lighter foods. Meat and other proteins increase metabolic heat production and could cause even more water loss.
Drink plenty of water, but avoid alcohol and caffeinated drinks.
Spend more time in air-conditioned places. If you don't have an air-conditioner in your home, go to a library, store or other location for part of the day to stay cool.
Avoid getting too much sun. Sunburn can reduce your body's ability to release heat.