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The term “heat island” describes built up areas that are hotter than nearby rural areas. The “urban heat island effect” (UHIE) describes the phenomena where temperatures in densely populated cities can be significantly higher than in surrounding areas. As U.S. EPA reports, the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.