This cross section through a typical city shows how temperatures are usually lower at the urban-rural border than in dense downtown areas.
Click on image for full size
Lisa Gardiner / Windows to the Universe, based on a figure from the Lawrence Berkeley National Laboratory
The Urban Heat Island Effect
An urban heat island is a metropolitan area that is significantly warmer than nearby rural areas. Many cities have air temperatures 2 - 5°C (3.6 - 9°F) warmer than the surrounding natural land cover. This temperature difference usually is larger at night than during the day and larger in winter than in summer. It is most apparent when winds are weak.
The changes made to the land surface in urban areas have a large impact on whether a heat island forms. For example, many cities have fewer trees than surrounding rural areas. Trees shade the ground, preventing the Sun’s radiation from being absorbed. Without them, the ground surface heats up. Fewer plants also mean that less evapotranspiration occurs, a process that cools the air.
Buildings and pavement also have a large impact on urban heat. As vegetation is replaced by roads and buildings, albedo is reduced and more heat is absorbed rather than reflected. Dark rooftops and dark pavement absorb more radiation. Tall buildings trap heat in their walls as well as their roofs. Tall buildings can also be barriers to wind, which would help move hot air away from a city.
Automobiles, which make heat from their engines and exhaust, also contribute to the heat island effect. Air pollution in urban areas can strengthen an urban heat island by trapping heat.
Some people have speculated that the growth of urban areas over the last century has caused more urban heat and that has caused global warming. However, this is not the case. While the urban heat island effect is important on a local scale, there is no evidence that it biases trends in the global historical temperature record. According to the Intergovernmental Panel on Climate Change, global warming is very unlikely to be influenced significantly by increasing urbanization.
An urban heat island can increase the magnitude and duration of a heat wave. And city heat can influence the weather too - changing wind patterns, clouds, and precipitation.
Today, many cities are making an effort to combat the urban heat island effect. White or reflective materials are being used for roofing and roads. Trees are being planted along city streets. And, in many areas, green roofs - living plants on rooftops – are being installed.
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