Urban Heat Islands/Cool Communities


Within the last century, California experienced tremendous population growth that resulted in an increased demand for land and metropolitan services near urban centers. The shift from a rural to an urban lifestyle created cities with vast road systems used by numerous vehicles, commercial and industrial buildings, and residential houses that dramatically altered the natural landscape; water, soil, and vegetation. As a consequence of replacing vegetation with dark colored surfaces, city temperatures are estimated by scientists to be 2° F-10° F higher than the surrounding environment. This unusual phenomenon is referred to as the “urban heat island effect.”

The lack of plant material and the traditional use of dark colored surfaces such as asphalt, are two of the largest contributors to urban heat. Unlike light colored surfaces that reflect solar (ultraviolet) radiation and trees that cool air temperatures, dark colored surfaces absorb incoming solar radiation, eventually reradiating it as heat energy that warms the local air temperature. In addition, vegetation provides cooling through a process called evapotranspiration and by shading surfaces.

Higher temperatures speed the formation of smog, increase heat related illnesses and mortality, raise demand for energy, and intensify the level of stress to the health of plants and wildlife.

Since its inception, the EAD has taken a role in curbing urban heat production through its many air quality and tree planting projects. In the last 10 years, the EAD has secured funding for the planting of 20,000 additional trees throughout the City.

In 2000, EAD began implementation of the "Cool Green Communities" urban forestry project. Through this demonstration program, the EAD planted over 1,000 trees along parkways and other public locations and erected a vine-covered structure to shade parked cars (shading parked cars decreases the temperature of the car, decreasing rate of gasoline evaporation and the resultant release of hydrocarbon air pollutants).

Since 1990, the combined efforts of City of Los Angeles departments, in partnership with local community-based organizations, have resulted in over 80,000 planted trees at schools, on City streets, in parks, at residences, and in local forests. Over the next several years, the City is committed to planting an additional 200,000 trees.

In addition to tree planting mitigation programs, the City is committed to energy efficiency by adopting a policy in 2001 requiring Energy Star® compliant roofing materials be used on all City facilities with new or replacement roofing, accounting for approximately 800 roofs. The City also is developing a demonstration site that will include light colored roofing and pavement, and shade trees to examine the efficiency of these strategies.

The Environmental Affairs Department has published a paper in the proceedings of the North American Urban Heat Island Summit in May 2002. The paper is entitled, "Los Angeles' Experience in Quantifying the Benefits of and Implementing Strategies to Mitigate the Urban Heat Island Effect."

For more information on the urban heat island effect visit:

U.S. EPA's Urban Heat Island Reduction Initiative

NASA & EPA's Urban Heat Island Pilot Project

Lawrence Berkeley National Laboratory Heat Island Group

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