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Global warming isn't the only reason cities are sweltering across America: cities themselves are creating more heat. In Los Angeles, at least climatologists report that while average daytime temperatures have risen three degrees over the last century, average nighttime temperatures are up by seven degrees. The reason is that features of the ever-expanding urban environment like golf courses, shopping centers and lawns trap the day's heat, and release it again after dark - keeping the atmosphere from cooling down. No reason to think the same thing isn't happening in other big cities.