Temperatures Rise as 'Heat Dome' Hovers Over Country

A double rainbow appears at sunset on Thursday, July 21 in Saukville, Wis.

A heat alert grew Wednesday to include parts of 21 states as a "heat dome" is expected hover over much of the nation later this week, with some places forecast to reach up to a dangerous 115 degrees.

Jim Darnell, of Darnell Home Improvement, waits in the shade as a co-worker cuts another piece of siding for a home improvement job on July 21, 2016, in Quincy, Ill.

Outdoor workers had to take more breaks than usual with the temperature in the upper 90's and the heat index in the 100s.

Children play in a water fall at the Crown Fountain in Chicago's Millennium Park, seeking temporary relief to the Midwest's excessive heat on July 21 in Chicago.

The "heat dome," will push conditions to their hottest point so far this summer, though record hot temperatures are not expected, according to the National Weather Service.

A lifeguard, center in red, overseas a platform as people beat the oppressive heat and humidity on July 21, at Lake Harriet in Minneapolis.

The heat dome will first hit parts of the central U.S. during the latter half of the week and will then spread toward the Northeast and mid-Atlantic late this week into the weekend.

A child cools off at the Alaskan Adventure splash pad at the Henry Doorly Zoo in Omaha, Neb., on July 21, as high temperatures and humidity affect much of the central U.S., making it feel as hot as 115 degrees Fahrenheit in some places and leading some cities to open cooling stations and take other precautions.

Patti Glessner, 52, of Cambridge, Md., relaxes on her raft at Keen Lake in Canaan Township, Pa.

The prolonged heat, plus the poor air quality, can quickly become dangerous — or even deadly — for those with respiratory issues or anyone out in the sun or in non-air-conditioned places.

Isaac Howard, 5, plays in a water feature during a visit with his family to the splash pad at Elver Park in Madison, Wis., on July 21.

The danger with heat domes is the prolonged nature of it. Although it won't reach record-high temperatures its constant days full of heat which will continue into the nighttime that don't give people a chance to really cool off.

The sun sets beyond visitors to Liberty Memorial on July 21.