Physics of the UHI Phenomenon
The energy balance ambient air-Earth surface is governed by energy gains, losses and storage and can be explained by the following relation:
Rn = [K↓ – K↑] – [L↓ – L↑]
Equation 1 Global energy balance
Rn is the net radiation reaching the surface; K↓ is the incoming short-waves depending on the geographical coordinates; K↑ is the outgoing short-waves depending on the surface albedo; L↓ incoming long-waves depending on the sky temperature; L↑ outgoing long-waves depending on the thermal properties of the surfaces reached.
Taking into account the correlation between the parameters in equation 1 and their dependence on other physical characteristics, the above equation can be also written in the following way:
Rn = (1-α) (K↓) + (L↓- ε σTs4)
Equation 2 Energy balance at the surface
Equation 2 explicates the dependence of the outgoing short-waves on the surface albedo (α) and the outgoing long-waves on the surface temperature (Ts) and the emissivity (ε) and σ that is the Stefan-Boltzmann constant.
The net radiation reaching the surface generates a heat gain (energy imbalance) that is transferred as follows:
Rn + A = λE + H + G + T +….
Equation 3 Energy balance
Where A is the anthropogenic heat; λE is latent heat; H is the sensible heat; G is the heat transferred to the substrate and T is the advective transport of heat. The latent heat is mainly due to the evapo-transpiration processes and it depends on the vegetation abundance in the detected area. The sensible heat depends on the thermal and optical characteristics of the surfaces; for example a high-reflectance surface is able to scatter solar radiation avoiding an extreme increase in the surface temperature. Other factors can be involved in equation 3 but usually are minor. G is mainly dependent on the thermal properties of the materials. A is referred to the human activities among the others, for example, road traffic, use of fuels and heat releases due to cooling system use in summer. In urban context equation 3 is mainly constituted by the sensible heat and the anthropogenic heat:
Rn + A = H
Equation 4 Energy equation in urban sites
Besides, for the rural areas, equation 4 becomes:
Rn + A = λE + H + G
Equation 5 Energy equation in rural sites
Equation 3 is an energy imbalance because not all the energy reaching the urban surfaces is re-radiated; but some of it is absorbed by the building materials and then transformed. That equation depends mainly by the characteristics of the building materials; in rural sites the amount of latent heat is consistently higher than that in an urban site. In an urban context because the surface albedo is lower than that in rural site, the surface temperature is higher. The surface temperature describes the urban heat island as a diurnal phenomenon connected to the solar radiation reaching the surfaces, but the stored heat is also released after the sunset. In this case the UHI can be detected and measured as a difference of air temperatures between an urban and a rural context. The heat released mainly depends on the thermal characteristics of the materials such as emissivity. While, the released heat dissipation depends on the canopy layer, in other words, on the local urban morphology and in particular on the strongly pronounced 3D structure (Santamouris, 2006).
UHI Seasonal and Diurnal/Nocturnal Behavior
UHI is characterized by significant spatial and also temporal variability. The greatest UHI is evident under conditions of clear sky and low wind speed, when the highest quantity of solar radiation is able to reach the urban surfaces and to heat them. Then, the low wind velocity – typical of the densely urbanized environments – decreases the amount of dissipated heat determining an accumulation of heat under steady-state conditions (Landsberg, 1981). The heat accumulated during the day is released after the sunset; the lower is the skin temperature the bigger is the amount of heat released. In particular in the two or three hours following the sunset the most quantity of heat is released by the building materials, determining the highest difference of temperature between the urban and rural sites. Moreover, the UHI can be observed both in summer and in winter, even if the impacts of the local warming during the summer are stronger than those during the winter (Santamouris, 2006).
UHI can produce secondary effects on local climate such as the increase in cloudiness and fog for the higher evaporation in correspondence of rain precipitation. The increase in cloudiness and air humidity gives rise to additional shower and thunderstorm (van Heerwaarden & Vilà-Guerau de Arellano, 2008).
Landsberg, H. E. (1981). The Urban Climate (Vol. 28). New York: International Geophysics Series
Santamouris, M. (2006). Environmental design of urban buildings. An integrated approach. London: Earthscan
van Heerwaarden, C. C., & Vilà-Guerau de Arellano, J. (2008). Relative humidity as an indicator for cloud formation over heterogeneous land surfaces. Journal of the Atmospheric Sciences, 65, 3263-3277
* Rearranged text from: Susca, T. (2011). Evaluation of the Surface Albedo in a LCA Multi-scale Approach. The Case Study of Green, White and Black Roofs in New York City. Ph.D. Thesis