The summer urban heat island (UHI) effect is a daytime elevation in the outdoor urban air temperature that results in part from the replacement of trees and other vegetation with buildings, roads and other heat-absorbing infrastructure.
The properties of urban roofs and pavements, as well as human activity, contribute to the formation of summer [no-glossary]urban heat islands[/no-glossary]:
- Urban surface properties. Roofs and pavements can constitute about 60% of the surface area of a U.S. city. These surfaces are typically dark in color and thus absorb at least 80% of sunlight, causing them to get warmer than lighter colored surfaces.1 These warm roofs and pavements then emit heat and make the outside air warmer.
- Human activity. Air conditioning, manufacturing, transportation, and other human activities discharge heat into our urban environments.
Urban heat islands can negatively affect the urban community and the environment.
- Increased energy use. Warm temperatures in cities increase the need for air conditioning (A/C) to cool buildings. This elevated demand can strain the electrical grid on a hot summer afternoon, making it more susceptible to brown-outs and black-outs.
- Impaired air quality. Warmer air accelerates the formation of smog (ozone) from airborne pollutants like nitrogen oxides and volatile organic compounds. Elevated demand for cooling energy in the form of A/C use can also increase the emission of air pollutants and greenhouse gases from fossil-fuel power plants.
- Illness. Higher air temperatures and lower air quality can aggravate heat-related and respiratory illnesses, and also reduce productivity.
1 Akbari H, Menon S, Rosenfeld A. 2009. Global cooling: increasing world-wide urban albedos to offset CO2. Climatic Change, 94 (3-4), 275-286.