Urban heat island

An urban heat island (UHI) is a built up area that is hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1–3°C warmer than its surroundings. [1] The temperature in Atlanta is 5 to 8 degrees Fahrenheit higher than outlying areas, and this excess heat produces increased rainfall and thunderstorms. [2]

This temperature gradient can gradually increase; for example, from 1945 to 1990 San Antonio has apparently gotten 3°C warmer than nearby New Braunfels. [3]