Catalina McCallum, right, and her brother, Jack, both from Greenwich, Conn., cool off in one of several CTA cooling buses stationed at the Lollapalooza concert in Grant Park Sunday, July 24, 2005, in Chicago.
AP/Chicago Tribune
The Midwest and Southwest are getting the worst of things, but it's more than hot in over a third of the U.S., including the central Mississippi valley, the Southeast, Texas and parts of California, with temperatures topping 90 degrees and in many cases soaring past 100.

The weather - in some areas combined with drought - has prompted many cities to open cooling centers and take other measures to save lives in what can be killer heat.

So far three deaths in Chicago are suspected as heat-related, and in Arizona, as of Saturday, at least 21 people had died, many of the Arizona victims were homeless, prompting officials to take action.

"Unfortunately for the homeless population that is not used to it, that's where the deaths occurred. That's why Phoenix went out of its way to make sure it focused on those in need," Phoenix Mayor Phil Gordon told CBS News' Early Show co-anchor Hannah Storm.

"We have so much heat across such a large portion of the country - even Las Vegas last week got to 117 degrees - all these temperatures which are above normal in many spots," says CBS News Meteorologist Michael Stroz. "That happens every few years. Some say that records are made to be broken, so we're just breaking them... Some meteorologists will say it's global warming, others will say it's just part of the cycle."

Sunday afternoon, temperatures at Chicago's Midway Airport had reached 104 degrees, just one degree lower than the highest temperature ever recorded in the city.

Other parts of the Midwest also reached triple-digit temperatures. Temperatures hit 102 degrees in St. Louis and 101 in Iowa City, Iowa.

The heat index - the combination of the heat plus the humidity - is even higher and calls for common sense precautions.

"For instance," says Stroz, "it may be 90 degrees, but when you factor in high humidities, it makes it feel hotter than it really is - sticky, real uncomfortable - that's when young children and the elderly have to take it easy - drink a lot of fluids, no alcoholic fluids, avoid strenous activity during the hottest part of the day."

In Chicago, sweat-drenched city workers have been checking on senior citizens Sunday and shuttled people to cooling centers as temperatures surpass the 100-degree mark here for the first time in six years.

The skyrocketing temperatures prompted Chicago officials to implement an emergency response plan that was honed after 700 people died during a July 1995 heat wave. An automated calling system began contacting 40,000 elderly residents at 9 a.m. to inform them about the heat.

"We're taking these steps because it is so hot and it's necessary for us to provide assistance to those who are not in a position to be able to help themselves to stay out of the heat," says Illinois Gov. Rod Blagojevich. "We learned ten years ago what could happen with stifling heat."

"If you looked at who died in 1995, it was not triathletes, it wasn't people at ballparks, it wasn't people at outdoor festivals, it was the elderly who were living alone," said Dr. William Paul, acting commissioner of the city's Department of Public Health.

The city also sent buses to act as air-conditioned shelters for some of the tens of thousands of fans who had gathered for the two-day-long Lollapalooza rock musical festival.