What is an Urban Heat Island?

On warm summer days, the air in urban areas can be 2 to 9 degrees Fahrenheit warmer than in surrounding areas. Scientists refer to these places of elevated temperatures as Urban Heat Islands (UHI), because the areas appear to be an island of heat within cooler surroundings.  A UHI can be a city or town, a housing subdivision, a roadway or a parking lot.

These higher temperatures contribute to air pollution, higher energy costs, water quality deterioration and a loss of other quality of life features. There is evidence that UHI’s contribute to global warming. Roads, buildings, and other hard surfaces absorb and retain heat leading to surface temperatures 50 to 70 degrees hotter than the air.  The removal of trees as areas are developed also contributes to the heating effect.

Planting trees is a simple and effective way to reduce the UHI effect. Creating green spaces in parks and communities, establishing shade trees along roadways, and planting on private property can assist in bringing down surface temperatures. 

• Trees absorb heat and cool the air through evaporation of water on leaf surfaces. 

• Trees shade man-made surfaces preventing the initial heating and storage of heat.

• Trees help to reduce storm water runoff and diminish flooding.

• Trees can make homes and buildings more energy efficient. 

• Trees have been used to cool homes for hundreds of years.

Illustration Courtesy of the U.S. Environmental Protection Agency

 - Terry Galloway
Treemendous Coordinator / Volunteer Services
Maryland Department of Natural Resources

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