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. Urban heat island effect is most noticeable during the summer and winter.
The main cause of the urban heat island effect is the modification of land surfaces. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature.
Monthly rainfall is greater downwind of cities, partially due to the urban heat island. Increases in heat within urban centers increases the length of growing seasons and decreases the occurrence of weak tornadoes.