<P> An urban heat island (UHI) is an urban area or 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 . UHI is most noticeable during the summer and winter . The main cause of the urban heat island effect is from 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 . The less - used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area . </P> <P> Monthly rainfall is greater downwind of cities, partially due to the UHI . Increases in heat within urban centers increases the length of growing seasons, and decreases the occurrence of weak tornadoes . The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams and put stress on their ecosystems . </P>

The tendency of urban areas to be warmer then the surrounding rural areas are caused by