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For more than 100 years, it has been known that two adjacent cities are generally warmer than the surrounding areas. This region of city warmth, known as an urban heat island, can influence the concentration of air pollution. The urban heat island is formed when ....

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.... stopped by the tall building walls that do not allow infrared radiation to escape as readily as do the relative level surfaces of the surrounding countryside. The slow release of heat tends to keep city temperatures higher than those of the unpaved faster cooling areas. On clear, still nights when the heat island is pronounced, a small thermal low- pressure area forms over the city. Sometimes a light breeze, called a country breeze which blows from the countryside into the city. If there are major industrial areas along the city's outskirts, pollutants are carried into the heart of town, where they tend to concentrate. At night, the extra warmth ....