Because urban areas are generally devoid of significant vegetation, the concrete and other construction materials for buildings and roads prevent the heat from the Sun from being absorbed. Instead, surfaces become hotter and drier. Cities and towns become warmer than surrounding rural areas to the point where, on a warm summer day, surfaces such as sidewalks and roofs can be as much as 50° to 90°F (27 to 50°C) hotter than the surrounding air. The warming effects are especially pronounced during the day, but the temperatures are also affected at night.

According to the U.S. Environmental Protection Agency, a city of about one million people can cause the surrounding atmosphere to heat up as much as 22°F (12°C) more than it would be under similar weather conditions in a rural area. Annually, the effect would be that the overall temperature for a city that size would be 1.8 to 5.4°F (1 to 3°C) higher than in the surrounding areas. There are several problems that result from heat islands: people tend to use their air conditioners and other utilities more, thus increasing energy consumption; this leads to more pollution, including greenhouse gases; these pollutants also affect human health; finally, rainwater that has fallen onto heated surfaces on pavement and roofs flows into sewers, and then into the environment, where the heated water affects wildlife.