 115 degrees or more for a fifth straight day. According to the National Weather Service, the city hit 119 degrees Tuesday and reached 120 degrees Sunday for the first time in history. A rep for the National Weather Service calls this the most extreme heat wave in the history of record keeping in Las Vegas since 1937. More than 142 million people across the U.S. are under heat alerts, many out west where the duration of the heat has been a major concern.