An urban area where the temperature is commonly 5–10°F (−15–−12°C) warmer than the surrounding countryside. Effects include reduced heating but higher air conditioning bills. Within cities, this warmth causes spring to arrive one to two weeks earlier and fall foliage to appear one to two weeks later. The chief cause is the high energy use within cities, which ultimately ends up in the atmosphere as waste heat. Also important, however, are the darker surfaces within cities, which absorb more heat, the relative absence of vegetative cooling through transpiration because of fewer trees, and less wind because of building obstructions.