The heat wave appears to have slowed to an unbearable crawl across the Great Lakes and Northeast, and no relief from the triple-digit temperatures is expected until Saturday.

Half of the United States is stuck with dangerously high temperatures between 90 and 100 degrees Fahrenheit (32.2 and 37.7 degrees Celsius), according to the U.S. National Oceanic and Atmospheric Administration (NOAA). At least 140 million people are under heat advisories, according to NOAA.

On July 20, 115 records were either tied or broken for highest temperature on that date, according to the National Climatic Data Center (NCDC) in Asheville, N.C. In Chicago, July 20 was the city's first 100-degree day in six years.

Making matters worse, there's little relief once the sun sets. Many places are stuck with high overnight temperatures, said Jake Crouch of the NCDC. On July 20, 322 records were either tied or broken for highest minimum temperature on that date, according to NCDC data.

These high lows mean that "there's not a lot of time for people, animals and plants to have a reprieve from the days' hot temperatures," Crouch told OurAmazingPlanet.

This is especially true in places that had a wet winter and spring, such as in the Great Lakes and upper Midwest, where rivers have swollen to record levels.

"There's a lot of moisture in the air, so that doesn't allow anything to cool down at night," Crouch said.

Then the next day, the moisture is still hanging in air, which creates the stifling humidity that helps raise the heat index. The heat index — a combination of heat and humidity that describes what the air temperature feels like — has been through the roof across the heartland during the recent heat wave.

A large ridge of high pressure has settled over the central and eastern United States, which will continue to push the heat index well above 100 Fahrenheit (37.7 Celcius) over a large portion of the nation, according to the National Weather Service (NWS).The NWS will issue heat alerts when the heat index is expected to exceed 105 to 110 Fahrenheit (40.5 to 43.3 Celcius).

Already 22 people have died from the hot weather this year, though that number could increase as other deaths are determined to be heat-related. During the 1995 heat wave in Chicago, one of the worst in recent history, more than 750 people died.

The core of this year's most extreme heat has been centered over Oklahoma and northern Texas. At Oklahoma City's Will Rogers World Airport, 28 days this year have hit at least 100 Fahrenheit (37.7 Celcius). The city could be on track to break its all-time record of 50 days at or above 100 Fahrenheit set in 1980.

That record is not close to the worst heat wave of all time, however. That record is claimed by Marble Bar, Australia. From Oct. 31, 1923 to April 7, 1924, (summer in the Southern Hemisphere) the tiny town scorched with 160 consecutive days over 100 Fahrenheit, according Australia's Bureau of Meteorology. In the United States, Death Valley has come close to Marble Bar's record, with 154 consecutive days of at least 100 Fahrenheit, in 2001.

This article was reprinted with permission from OurAmazingPlanet.

Related on OurAmazingPlanet: