Urban heat island effect over a city relative to buildings and greenspaces with trees.
The conversion of rural land for urban use is referred to as urbanization (Miller, 1997). Seventy-four percent (203 million) of the U.S. population in 1989 (Frey, 1984) lived in urban areas, and populations are slated to increase to greater than 80 percent by 2025 (Fox, 1987).
Concentrated development in urban areas can create an “island” of heat surrounded by a “sea” of cooler rural areas. This is called the urban heat island. Buildings, asphalt, and concrete absorb solar energy, thus resulting in the emission of long-wave radiation that heats the atmosphere (air) in cities. Cities also use large amounts of energy, and emit this energy as waste heat, adding to the urban heat island effect.
The urban heat island is an effect of urbanization in forested or farmland regions, causing the temperature in urban areas to be higher than the surrounding rural areas. This is especially true during calm and cloudless nights. Figure 1 is a schematic of a heat island profile for a city, illustrating temperature differences as you migrate from the rural periphery, peaking in the city center. In desert environments, cities planted with urban forests can result in an opposite phenomenon of creating an “urban oasis” effect.
- R. W. Miller, Urban Forestry: Planning and Managing Urban Greenspaces. 1997, Long Grove, Illinois: Waveland Press Inc.
- H. T. Frey, Expansion of Urban Area in the United States: 1960-1980. U.S.D.A. Economic Research Service Staff Report No. AGES830615. Washington, D.C. (1984)
- R. Fox, Population Images. United Nations Fund for Population Activities, 1987, New York.
- “Cooling Our Communities – A Guidebook On Tree Planting and Light-Colored Surfacing”. US Environmental Protection Agency 22-P-2001, January 1992. Link: http://www.epa.gov/hiri/ for Smart Growth and Urban Heat Islands (PDF).
By: Sharon Jean-Philippe, Assistant Professor, Urban Forestry, University of Tennessee, and Bill Hubbard, Regional Forester, Southern Region, University of Georgia