We were in las Vegas and Hoover Dam in July five years ago. Since we live at the northern edge of the Chesapeake the humidity here is terrible so I always believed the dry heat saying, but it was 115 degrees, and it hit you like a slap in the face the minute you set foot outside. We drove to the Grand Canyon and the temperature dropped 40 degrees, it was amazing. There are obviously limits to the benefit of low humidity. We are going to Vegas this May to drive to Bryce and Zion national parks, that was our 25th anniversary trip and this is our 30th. I hope the weather is more moderate in May than July.