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What is The Heat Island Effect?
A metropolitan area which is significantly warmer than its surroundings is defined as an “urban heat island” (UHI). As population centers grow in size from village to town to city, they tend to have a corresponding increase in average temperature. The Environmental Protections Agency (EPA) notes that, “On hot summer days, urban air can be 2-10°F [2-6°C] hotter than the surrounding countryside.
Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure. These changes contribute to higher urban temperatures in a number of ways:
- Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and leaves (evapotranspiration).
- Tall buildings and narrow streets can heat air trapped between them and reduce air flow.
- Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect.
- Dark surfaces commonly used in roofing and paving materials absorb the heat from sunlight rather than reflect the sunlight and the resulting heat. They also require greater energy usage during the nighttime hours to illuminate driving and walking areas.
While there is some wintertime benefit to the “heat island effect”, “the harmful impacts from summertime heat islands are greater than the wintertime benefits”. (EPA) “The negative effects of summertime heat islands” include, “increased levels of air conditioning demand, air pollution, greenhouse gas emissions, and heat-related illness and mortality, as well as reduced thermal comfort.” Thus for millions of Americans living in and around cities, heat islands are of a growing concern.
Related Links:
Encyclopedia of Earth: Heat island
CNN - Atlanta Heat Island Project
Learning about Heat Islands - from the Heat Island Group
Urban Heat Islands: Hotter Cities By James A. Voogt
NASA Earth Observatory: The Earth's Big Cities, Urban Heat Islands
Research and mitigation strategies on UHI - National Center of Excellence on SMART Innovations at Arizona State University
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