Urban heat island

An urban heat island occurs when the infrastructure of a city uses up energy, thus making an "island" where it's warmer than the city's surrounding rural areas.

On a typical summer day, the sun can heat exposed surfaces in the city, like pavements and rooftops, to temperatures 50 to 90 degrees hotter than the air. Surfaces in rural areas, which are generally more shaded and moist, stay close to air temperatures. Surface urban heat islands are hottest during the day.

On the other hand, atmospheric urban heat islands, where the air is warmer than its surroundings, are strongest at night because of the time it takes the release of heat from the infrastructure to reach the atmosphere. In a city of 1 million people, the air can be 22 degrees higher than its surroundings.