Urban areas have ‘heat islands’, but the desert is another animal, and there is a lot of desert in the western US.
Unlike, say, the Saharan desert, which is bare sand and mobile dunes, American desert floors are generally shaded with (rather noxious) vegetation, adapted to survive droughts common to an area that may last decades or centuries. Sand and scattered rock outcroppings are poor heat sinks which is why there is such a drastic, at times 50 degree, temp change between day and night in the desert. A camper may wake up shivering at 5am but by 9am it will be 100 degrees or more in the summer, too hot for rattlers to survive exposed.
https://www.worldatlas.com/articles/the-major-deserts-of-the-united-states.html