HEAT ISLAND

A localized high temperature over a city, higher than the surrounding region. The temperature perturbation compared to the surrounding countryside is typically +1° C for small cities to +4° C for larger cities. This higher temperature is due to the replacement of natural vegetation in the city with asphalt, concrete, buildings, and other human-made-structures. These human-made structures typically have low albedo, a high heat capacity, and no evapotranspiration., so that most of the heating is sensible. This has been studied in cities, such as St. Louis, Missouri, where it was found to cause large increases in the amount of hail thunderstorms, and rainfall downwind of such cities. Columbia, Maryland, however, has a heat island of only +0.5° C, because the city was developed without eliminating much of the natural vegetation.