Analysis of the Urban Heat Island Spatial Variability over Tehran

The Urban Heat Island (UHI) effect refers to the temperature rise of any man-made area. It is a phenomenon in which cities become warmer than the surrounding suburbs. In other words, there is a temperature difference between the cities and the area surrounding them. Generally, the UHI effect could be a result of excessive and unplanned growth of urbanization. The behavior of artificial urban texture in terms of absorption of short-wave and long-wave radiation, transpiration, releasing of anthropogenic heat, and blocking prevalent wind is significantly different from that of the rudimentary nature. Surface geometry on the other hand, decreases wind speed in urban regions which plays a great role in formation of UHI. As the energy balance inside a city alters, UHI intensity varies. This means that UHI intensity is not spatially and temporally similar in different cities. Also it must be noticed that UHI formation in a city usually has diurnal or seasonal patterns which are mostly affected by synoptic weather conditions. There are three main synoptic and local climatology parameters affecting UHI formation: air pressure systems, cloudiness, and wind speed. Under stationary high-pressure system conditions temperature differences between urban and rural areas become large. UHI intensity is largest in calm air and cloudless sky conditions and tends to disappear in cloudy and windy weather.