Urban heat island effect

An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. As population centres grow in size from village to town to city, they tend to have a corresponding increase in average temperature, which is more often welcome in winter months than in summertime. The EPA says: "On hot summer days, urban air can be 2-10°F [2-6°C] hotter than the surrounding countryside. Not to be confused with global warming, scientists call this phenomenon the 'urban heat island effect'"

source: wikipedia