Fitz and Jontu, two Indian rhinos at the Henry Doorly Zoo in Omaha, Neb., cool off in a mud pit, Wednesday, July 20, 2011. The heat "dome" sitting over much of the U.S. is caused by a huge area of ... more 
Fitz and Jontu, two Indian rhinos at the Henry Doorly Zoo in Omaha, Neb., cool off in a mud pit, Wednesday, July 20, 2011. The heat "dome" sitting over much of the U.S. is caused by a huge area of high pressure that's trapping and compressing hot, humid air from the Gulf of Mexico beneath it and pushing the jetstream, and thus cooler, drier air, far northward into Canada. (AP Photo/Nati Harnik) less 
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Associated Press | Photo By Nati Harnik
Wed, Jul 20, 2011 5:13 PM EDT