Causes of the Urban Heat Island Effect

The urban heat island effect is best understood by first considering the basic energy-balance equation of a surface. This equation is given by

R_S + R_L + A = R_N + A = LE + H + G

where R_S = total shortwave radiation, direct and diffuse, absorbed at the surface, R_L= net longwave radiation at the surface, A = energy from anthropogenic sources, R_N= total net radiation, LE = latent (energy due to evaporation) heat flux, H = sensible (convective) heat flux, and G = downward heat flux due to conduction of the surface.

During a cloudless summer day, solar radiation (shortwave) is received at about 500W/m², and is the primary cause of surface heating. Although this radiation is ubiquitous at the surface, the actual amount of energy absorbed varies with different materials and is determined by the material's albedo. Albedo measures a material's reflective capacity, and ranges from 0 to 1. An albedo = 0 refers to a perfect absorber whereas a material with albedo = 1 is a perfect reflector. Hence, materials with low albedos absorb more incoming solar energy and consequently have higher surface temperatures than materials with high albedos.

The longwave radiation component, R_L, depends on the balance of energy leaving and entering the surface. Longwave radiation entering the surface is due primarily to radiation that has been absorbed and reemitted downward by the atmosphere. Similarly, the surface itself radiates longwave energy upward as a function of surface temperature.

The net radiation absorbed at the surface is then channeled into the three fluxes given by LE, H, and G. The sum of these three fluxes must remain constant, so that an increase in one necessarily decreases one or both of the other two. Conversely, a decrease in one will result in an increase in the sum of the other two fluxes. Specifically, changes in the sensible heat component, H, are especially important because the urban heat island effect measures differences in air temperatures.

The causes of urban heat islands are well established and include vegetation removal from urban areas, impermeable man-made surfaces, large buildings, air pollution, and anthropogenic activity (Akbari et al., 1992). These causes are easily understood by keeping in mind the energy balance equation.

In natural environments heat storage during the day is moderated by evaporation of water from both soils and plant transpiration. These processes, collectively called evapotranspiration, require energy input and result in overall lower air temperatures. In urban areas, however, much of the natural vegetative cover has been removed and soils have typically been covered with concrete, asphalt, and other man-made materials. Consequently, urbanized areas often lack the moisture necessary for latent energy transfer and an equivalent amount of energy is then channeled into ground storage and sensible heat flux. This results in higher urban temperatures (Garbesi, Akbari, Martien, 1989).

Man-made materials also result in urban heating because they typically have albedos that are lower than materials found in natural environments (Barry, Chorley, 1998. Oke, 1987). These low-albedo materials absorb more solar radiation than natural materials which in turn increases the net radiation component in the energy balance equation. This increase, coupled with minimal latent energy transfer, creates a larger sensible heat flux and therefore increases urban air temperatures.

Large buildings contribute to urban warming because they too absorb a large amount of solar energy during the day. In addition, within the urban canopy of large buildings the amount of space available for direct longwave radiation upward is reduced. Reducing this radiative space, called the sky view factor, results in the interception of outgoing longwave radiation rather then allowing it to escape into the atmosphere. Consequently, the net radiation component in the energy balance equation must increase. Again, this increase in net radiation causes an increase in latent, sensible, and storage energy fluxes.

Air pollution also effects the net radiation component in the energy budget equation. During the day, pollution in the form of particulates and chemical compounds both lowers incoming shortwave radiation from above and increases downward longwave radiation. These alterations in the energy balance at the surface result in a net radiation decrease and a net radiation increase, respectively, although the total effect is a slight decrease in net radiation receipt. At night, however, shortwave radiation receipt falls to zero and longwave emission downward becomes the dominant process. This 'blanket' effect due to pollution increases the nighttime heat island.

Finally, anthropogenic activity contributes additional energy to the energy balance equation. Modern cities are centers of especially high energy consumption when compared to natural areas due to automobiles, industry, and other human energy requirements (Odum, 1997), and this energy eventually dissipates into the urban environment as heat. Although anthropogenic heat sources do affect heat islands in winter and in high latitude cities, anthropogenic heat input during the summer and in mid to low latitudes is small when compared to solar shortwave radiation (Oke, 1987).

Although all of these factors affect the energy balance of urban surfaces, shortwave energy receipt easily is the dominant term. Consequently, urban heat islands are largest in areas where insolation varies significantly on both a diurnal and a seasonal basis. These areas are typically mid to low latitude and continental where the moderating effects of water bodies are not present and cycles of differential heating are well defined. For this reason, the severity of heat islands is mostly determined by the interaction of materials and vegetation in the urban landscape with

solar radiation receipt at the surface (Akbari et al., 1992).

Heat islands differ in magnitude and extent, but they generally share the same diurnal cycle shown for St. Louis, Missouri in Figure 1. During late morning, the heat island is apparent but maximum temperature differences between downtown areas and the surrounding countryside are only on the order of 1-2°F. By midday, insolation reaches a maximum, but the heat island is still relatively small because both the urban and rural areas are radiating large amounts of energy due to their high temperatures. Subsequently, the corresponding air temperatures are high in both areas. The story changes, however,once the sun has set and solar radiation receipt falls to zero. Now, as man-made materials release energy that has been stored throughout the day, urban areas cool off significantly slower than surrounding rural areas. This urban heating process continues through the early morning and results in maximum heat islands that occur at night rather than during the day when solar radiation receipt is greatest.