Urban Heat Islands
Urban regions are among the most rapidly changing environments on earth. As cities grow, they impact local and regional climates, including temperature averages and extremes. Urban areas are known to alter mean annual air temperatures by 2-5°F per 100 years and up to 20°F at night. Temperature changes affect urban dweller in many ways, influencing their health and comfort, energy costs, air quality and visibility levels, water availability and quality, ecological services, recreation, and overall quality of life.
For nearly 100 years, it has been believed that urban areas affect the local climate, mainly in terms of the temperature. The urban effect is due to changes in the thermal properties, moisture and aerodynamic character of the built environment. These changes create a distinct urban boundary layer, or heat dome. This heat dome extends vertically above the city, and in windy conditions can be located downwind as a plume. The temperatures within the heat dome can be 10°F (6°C) higher than the surrounding areas.
At a given time of day, a balance of incoming energy from the sun and outgoing heat from the surface determines the surface temperature. Solar radiation strikes the surface, and reflects a portion back to space and with the remainder both heating the surface and evaporating any water that may be present. The heat is transferred upwards, in part by thermal (infrared) radiation and by turbulence due to the wind flowing over the surface.
In built urban areas, there is generally less water on the surface, as compared to the outlying rural areas. In addition, the walls of buildings radiate horizontally instead of vertically, which traps the heat near the surface. Both of these factors result in the elevation of temperature that is the urban heat island.
This Virtual Organization provides high spatial resolution of urban climates, including data provided by citizen scientists. Additionally, we explore and present prior and on-going research into the environmental, economic and social impacts from the UHI effect as well as innovative mitigation strategies that include urban design, engineered materials, urban forestry and policies.
The United States Environmental Protection Agency directs the the Urban Heat Island Reduction Initative which provides useful resources, links and documents. http://www.epa.gov/hiri/index.html
