An urban heat island is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area, and increase in its average temperature. How far away does this effect persist? Does it significantly contribute to global warming?