Cool Your City: Urban Heat Island Effect Mitigation

Urban areas around the world have been found to have air temperatures 5-10 degrees F (2-5 degrees C) warmer than the surrounding countryside. This is a phenomenon known as the urban heat island effect.

Urban heat islands are caused mainly by replacing natural vegetation with concrete, asphalt, and similar materials that retain heat more effectively. In most areas, the urban heat island effect is most pronounced at night.

They are considered to be a problem for several reasons:

• Higher air temperatures increase the amount of energy used for cooling, raising cooling bills and contributing, in most regions, to air pollution, climate change, and foreign oil dependence due to the burning of fossil fuels.
• Higher air temperatures are dangerous for young children and the elderly. Higher temperatures in cities contribute to a rise in heat-related deaths.
• Higher air temperatures increase air pollution. Higher air temperatures encourage the formation of smog from nitrous oxide and other compounds emitted by cars, factories, and power plants. The amount of smog is directly proportional to the air temperature. Smog contributes to increased rates of respiratory problems, heart attacks, and other health problems, and is especially dangerous for young children, the elderly, asthmatics, and people with existing respiratory or coronary problems.

The urban heat island effect also effects water moving through or out of the city, increasing water temperatures and lowering the quality of marine ecosystems.

There are a number of ways you can fight the urban heat island at home and around your city.

• Planting trees. Trees are one of our most important allies in the fight against the urban heat island effect. Air temperatures directly under trees can be as much as 25 degrees cooler than temperatures over unshaded blacktop. Trees can be sited strategically to shade roofs, pavement, walls, and other surfaces, keeping them cooler and reducing energy bills. Trees also provide a cooling effect through evapotranspiration. Other vegetation, including grass, shrubs, and vines, also provides cooling effects, though not usually as significant as trees.
  
• White roofs. Dark surfaces absorb most of the sun's energy that hits them, while light surfaces reflect it. Well designed white or light colored roofs can significantly reduce the amount of heating that occurs through the roof, lowering the roof surface temperature by as much as 100 degrees and reducing heat transfer to the interior, reducing energy use. Light colored pavement, walls, and other surfaces also reduce the urban heat island effect.
  
• Green roofs. Another green roofing option is green roofs. Green roofs are roofs planted with grasses, flowers, shrubs, or other vegetation. Like trees planted at ground level, the vegetation shades the surface, keeping it cooler. Reducing the urban heat island effect isn't the only benefit of green roofs - they can also be used to grow fresh, local produce in the city, and they significantly reduce stormwater runoff as well. Living walls provide similar benefits.
  
• Permeable pavement. Many types of permeable pavement reduce the urban heat island effect in one or more ways. For example, many grid or block pavers incorporate crass or other groundcovers, which help shade the surface of the pavement and increase local evapotranspiration. Porous pavements often have much lower total mass than asphalt or concrete, reducing the amount of heat they absorb, and many permeable pavements are lighter colored than traditional pavement as well.