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Nearly half of U.S. population facing extreme heat. Excessive-heat watches, warnings and heat advisories were in effect in more than 30 states July 21st, in what the weather service described as "a large portion of the central U.S. and Ohio River Valley, as well as portions of the mid-Atlantic and northeastern states. Temperatures will feel like 100 to 110 degrees or higher during the afternoon hours."

The heat wave in recent days has brought heat index values -- which measure how hot it feels -- to as high as 131, the weather service said.

Heat indices on Tuesday reached 129 in Newton, Iowa; 121 in Taylorville, Illinois; 122 in Gwinner, North Dakota; and 123 in Hutchinson, Minnesota.

Minneapolis recorded its highest dew point ever, 82 degrees, on Tuesday. The dew point temperature is a measure of atmospheric moisture.

Part of what makes the heat wave so dangerous is that it offers no break, even at night. Chicago on Tuesday tied its record for the warmest low temperature of 78 degrees. In Rockford, Illinois, temperatures dropped down only to 76 degrees -- the warmest on record.

The Centers for Disease Control and Prevention said people in areas affected by extreme heat should drink more water than usual -- and not wait until they feel thirsty. "Avoid alcohol or liquids containing high amounts of sugar," the CDC says.

The CDC also recommends taking cool showers or baths and avoiding direct sunlight. "Check on those most at-risk twice a day," the CDC said. http://www.cnn.com/2011/US/07/20/heat/index.html

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