The Associated Press . Josh Kroes sits in a lawn chair under a shade tree in Racine, Wis., on Wednesday evening. Kroes took a T-shirt, soaked it in water and froze it for about five minutes.
CHICAGO -- If the extreme heat and humidity lingering over much of the nation feels like a steam bath, it's because the same principles are at work in the atmosphere.
Vast amounts of warmth and moisture have become trapped under a huge ''heat dome,'' bringing record-breaking temperatures and thick, tropical air to scores of cities from the Plains to the Ohio Valley. Now the system is moving east to spread the misery to some of the country's most densely populated areas through the weekend.
With temperatures hovering around 100, Jeff Grembocki and other construction workers prepared Wednesday to pour concrete for a walkway improvement project near downtown Kansas City. Empty Gatorade bottles lay strewn across their job site.
Grembocki said the heat saps his energy so much that he falls asleep soon after getting home. He only rouses for a couple of hours to watch TV before going back to bed.
''The air conditioning, when it hits you, it's all you can do to stay awake,'' he said.
The heat dome forms when a high pressure system develops in the upper atmosphere, causing the air below it to sink and compress because there's more weight on top. That raises temperatures in the lower atmosphere, said Eli Jacks, a meteorologist with the National Weather Service in Silver Spring, Md.
The dome of high pressure also pushes the jet stream and its drier, cooler air, farther north -- it's now well into Canada -- while hot, humid air from the Gulf of Mexico circulates clockwise around the dome, traveling farther inland than normal.
Combined with generally clear skies and the sun's higher summertime angle, ''it gets really hot,'' Jacks said.
The cruel result: eye-popping heat index readings measuring temperature combined with humidity. In Newton, Iowa, it was 98 degrees Wednesday with a heat index that made it feel like 115. A day earlier, Newton's heat index hit 129 degrees.
In Indianapolis, the thermometer read 98 degrees but it felt like almost 114. Chicago's Midway Airport recorded a high of 99 degrees, which felt like 108. Humidity levels in some of the hottest cities ranged from 40 to 60 percent.
The formation of the dome also explains why conditions in, say, North Dakota aren't much different this week than in Houston. The big difference is that people in Houston are accustomed to hot weather. Those in the north are not.
''In places where the highest temperature you ever expect is in the 80s and you're at 102, there are big health concerns,'' because fewer people have air conditioning or fans, Jacks said. ''Heat is the No. 1 killer out of all weather hazards.''
What's more, because of the humidity, even nighttime brings little relief.
Humidity makes the weather feel far hotter because the body, which cools itself by perspiring, has to work harder when the air is already moist.
''It's harder to cool down,'' said Jannie Ferrell, a National Weather Service meteorologist.