Remote Sensing and the
Urban Heat Island Effect
Urban heat islands, or UHI, come in two types; surface urban heat islands (surface UHI) or atmospheric urban heat islands (atmospheric UHI)
Surface UHI occur when dry, exposed urban surfaces are heated by the sun to temperatures 50 to 90 degrees Farenheit warmer than the surrounding air temperature. Surface temperatures are affected by solar intensity and other weather conditions, causing surface heat islands to be more prevalant in summer months. Atmospheric UHI occurs when the air temperature in an urban environment is hotter than the surrounding suburban or rural air temperatures. Annual mean air temperatures of large cities can be up to 5.4 degrees Farenheit warmer than its more rural surroundings. The air temperature difference in cities is most evident during nightime, when the urban infrastructure slowly releases the heat that was absorbed during the day causing air temperature differences of up to 22 degrees farenheit warmer compared again to the more rural surrounding. Although hotter surface temperatures do influence hotter air temperatures, the air mixing that occurs in the atmosphere means that the relationship between the two is not constant, as atmospheric temperatures vary less than surface temperatures. The following image shows hypothetical variations in surface and air temperatures across an urban heat island (http://www.epa.gov/heatisland/resources).
These urban heat islands are the result of many different factors, including reduced vegetation. Less trees means less shade, and more paved area means more runoff and subsequently less evapotranspiration, a process in which water evaporates off the surface and cools surrounding air temperatures. Other key factors in urban heat islands stem from the properties of the materials used in urban landscapes. Darker materials, such as those found on roads and rooftops, have a lower solar reflectance in the visible spectrum of incoming solar radiation. In addition, materials like stone and steel have higher heat capacities than sand or soil, causing the urban areas to absorb and store much more heat compared to rural areas (http://www.epa.gov/heatisland/resources).
For more detailed information on Urban Heat Islands, please see the links under Urban Heat Island in the Additional information tab above
Image source: www.epa.gov/heatisland