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In page Urban heat island:

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Some cities exhibit a heat island effect, largest at night. Seasonally, UHI shows up both in summer and winter.[11][12] The typical temperature difference is several degrees between the center of the city and surrounding fields. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports, as in "68 °F (20 °C) downtown, 64 °F (18 °C) in the suburbs". "The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4 °F (1.0–3.0 °C) warmer than its surroundings. In the evening, the difference can be as high as 22 °F (12 °C)."[13][14]