A city, like a good cast-iron pot, holds its heat.
The 50 largest cities in the United States are, in a year-round average, nearly one degree warmer than their natural environs. At night, or at certain times of the year, that disparity grows: Las Vegas in the summer, for example, is on average more than seven degrees warmer than the surrounding desert. The differential is lower in the morning and highest after sunset. A 2014 report from Climate Central on U.S. cities recorded an incredible maximum urban heat island differential of 27 degrees. (2015) by Henry Grabar on Next City.