Several factors contribute to the urban heat island phenomenon.  Urban materials, such as concrete, glass, and asphalt, have high heat storage capacities and so they store heat during the day and release it at night.  Additional “anthropogenic” heat, used to heat buildings, can escape from the sides of buildings, especially in the winter at higher latitudes where energy consumption is greatest. 

The irregular geometry of cities reduces wind speeds, and provides more surface area for the absorption of solar radiation.  Furthermore, pollutants in the urban canopy trap heat emitted from the urban area and reradiate it back toward the surface.  All of these factors contribute to a warming of the city environment.  Much of the urban area is typically paved or built upon.  This “waterproofing” of the city results in greater surface runoff, reduced moisture retention at the surface, and a decrease in the amount of cooling that is normally associated with the evaporation of moisture. 

An urban-rural temperature profile (Figure 1) illustrates the aptness of the “island” analogy; the peak in temperature in the core of the city protrudes from the “sea” of surrounding, cooler temperatures.  The exact form of the temperature profile depends on land use within the city.  Parks, for example (Figure 2), can help reduce the rural-urban temperature difference (DTu-r) by providing shade and a source of moisture that can enhance evaporative cooling. 

                                Figure 1: A stylized cross-section of a typical urban heat island (from Oke, 1978) 
 

Figure 2: The shady environment of parks serves to reduce the rural-urban temperature difference.  In urban environments, moisture is lost as runoff.  In parks, moisture retained in the soil enhances evaporative cooling.
 
 

• For information on Atlanta’s urban heat island, click below: http://svs.gsfc.nasa.gov/imagewall/LandSat/atlanta_heat_background.html 
• How to cool down the 2002 Olympic Games, click on the following: ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1998/98-195.txt
• To learn more about weather and climate click on the following:  http://www.cla.sc.edu/geog/faculty/carbone/geog202.htm

Contributed by Helen Power:  http://www.cla.sc.edu/geog/faculty/power.html  and Jarrett Laughlin:  http://www.cla.sc.edu/GEOG/students/Laughlin_Jarrett.html