13 Dead as Heat Wave Broils US Heat 'dome' creates pressure cooker-like conditions By Evann Gastaldo, Newser Staff Posted Jul 20, 2011 7:41 AM CDT 11 comments Comments A man lays down next to the fountains at Columbus Circle on July 19, 2011 in New York to try and beat the heat. (Getty Images) (Newser) – The heat wave currently plaguing the US has broken more than 1,000 heat records and claimed 13 lives, the BBC reports. Eighteen states from Montana to West Virginia are under a National Weather Service heat advisory, with temperatures reaching more than 90 degrees in many states and the heat index topping 100 degrees, and the high temperatures are moving east. In some regions, the heat index—a combination of temperature and humidity—reached as high as 131 degrees. Roads are cracking and water mains are rupturing in Oklahoma, and the US Border Patrol says more illegal immigrants are dying as they try to cross the border under these dangerous conditions. The AP explains that the US is baking in a heat "dome," which is like a pressure cooker. The dome, created by a large high-pressure area, compresses hot, moist air underneath it, thus creating the miserable conditions. Click for more on the phenomenon.