Cool Science: Urban Heat Islands

Urban Heat Island
A satellite image of Atlanta, Georgia during a summer day. Notice how the red splotches (indicating higher temperature) in the infrared picture (bottom) are clustered around downtown Atlanta. (Image courtesy of NASA)

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.

Causes

The properties of urban roofs and pavements, as well as human activity, contribute to the formation of summer urban heat islands:

  • 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.

Consequences

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.

Notes

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.

Related Publications

2014

2012

2011

2005

Akbari, Hashem, Ronnen M Levinson, William A Miller, and Paul Berdahl. Potentials of urban heat island mitigation. Proceedings of the 1st International Passive and Low Energy Cooling for the Built Evironment. Proceedings of the 1st International Passive and Low Energy Cooling for the Built Evironment, 2005. http://escholarship.org/uc/item/20j676c9.

2003

2002

2000

Akbari, Hashem, and Arthur H Rosenfeld. Cool communities. Macmillan Encyclopedia of Energy 1. Macmillan Encyclopedia of Energy (2000): 305-8.
Taha, Haider, Alan K Meier, Weijun Gao, and Toshio Ojima. Mitigation of urban heat islands: meteorology, energy, and airquality impacts. Journal of Architecture, Planning and Environmental Engineering 529. Journal of Architecture, Planning and Environmental Engineering (2000): 69-76. http://ci.nii.ac.jp/naid/110004657413/en.

1999

1998

1996

1990

Huang, Yu J, Susan Davis, and Hashem Akbari. A guidebook for the control of summer heat islands. 1990 ACEEE Summer Study on Energy Efficiency in Buildings. 1990 ACEEE Summer Study on Energy Efficiency in Buildings, 1990. http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5484512.

1988