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Heat islands occur when urban areas experience higher levels of thermal heating than adjacent exurban and rural areas. Cities that maintain a higher average temperature than their surroundings can be viewed as islands of heat surrounded by cooler, nonurban landscapes. Around the world, many cities maintain air temperatures up to 10 degrees F (5.6 degrees C) warmer than surrounding areas. Scientists, urban planners, and historians are beginning to recognize that urban heat islands are not simply inconsequential environmental phenomena. Rather, the effects of urban heat islands are now being linked to processes of urban economic development and to levels of individual and community vulnerability.
Urban areas contain distinct physical properties that contribute to higher surface temperatures. The conversion of formerly vegetated landscapes to nonvegetated surfaces “forces” urban temperatures upwards by changing the thermal properties of urban environments in four significant ways.
First, vegetation absorbs solar energy out of the atmosphere in order to complete the process of evapotranspiration. Removing vegetation therefore eliminates an important cooling mechanism. Second, materials commonly used in the construction of asphalt roads and parking lots decrease the overall reflectivity, or albedo, of cities while simultaneously increasing heat-absorbing capacity. Third, the conversion of urban areas to nonimpervious surfaces facilitates the expedient removal of rainwater from the urban system. Because the evaporative capacity of water takes heat out of the urban environment, its rapid removal minimizes this important cooling process. Fourth, the replacement of low-lying trees and buildings with large structures that block natural wind patterns can diminish the role of wind as a natural cooling mechanism.
Urban heat islands can have negative health and environmental consequences. Increased average urban temperatures, especially during hot spells, create health hazards for many vulnerable urban residents. The 1995 midsummer Chicago heat wave resulting in approximately 525 fatalities. Researchers noted that while many urban residents experienced only minor inconveniences when daytime temperatures hovered near 100 degrees for over 5 days, others, such as the elderly and homeless, fell victim to the unrelenting heat.
Urban heat islands also increase photochemical reactions that lead to the production of harmful, ground-level ozone pollution. In Los Angeles, health risks from high smog levels increase when temperatures rise over 95 degrees F. Moreover, as temperatures increase, demand for air conditioning by urban residents goes up, which in turn leads to increased coal-fired energy production and more pollution. Higher urban temperatures can also disrupt local weather patterns. The city of Atlanta continues to experience heavier rainfall, increased thunderstorms, and locally generated winds as the city expands and the heat island effect becomes more pronounced.
A consortium of U.S. organizations, including the National Aeronautics and Space Administration and the National Ocean and Atmospheric Administration, use thermal sensory data to detect “hotspots” and evaluate existing surface characteristics in U.S. cities. The U.S. Environmental Protection Agency suggests a number of “smart growth” approaches for cities, including reducing large parking lots and other impervious surfaces, maintaining preexisting vegetation, promoting tree planting programs, and establishing educational outreach efforts for urban residents. Many municipalities have initiated programs to mitigate the production and/or effects of heat islands. These include promoting green roofs, aggressively incentivizing carpools, building with lighter surfaces, and strategically designing urban corridors that maximize wind channeling.
- T.R. Oke, Boundary Layer Climates, 2nd (Routledge, 1987);
- Stone and M.O. Rodgers, “Urban Form and Thermal Efficiency: How the Design of Cities Influences the Urban Heat Island Effect,” Journal of the American Planning Association (v.67/2, 2001).