Estimation of the Urban Heat Island Effect

Cities are often warmer than surrounding rural areas, a phenomenon known as the urban heat island (UHI) effect.  The difference between urban and rural temperatures is associated with concentrations of building materials that store daytime heat and emit it at night, reduced vegetation, and heat from buildings, vehicles and urban inhabitants. In cities that are already warm seasonally and are increasingly subject to heat waves and other extreme weather events, urban heating may exacerbate thermal discomfort or heat-related illness.  Further, the UHI effect reduces opportunities to naturally ventilate buildings at times when temperatures would otherwise have been acceptably cool.  

With two goals in mind, we are studying this effect in several cities worldwide, using a range of meteorological models and urban energy balances.  First, we have developed software, the Urban Weather Generator, which allows designers to estimate the thermal impact of urban development on building performance.  Figure 1 shows the urban boundary layer and identifies the lowest portion, the urban canopy layer, where human activity takes place.  Figure 2 shows the two-way interaction between buildings, which alter the urban energy balance, and the urban atmosphere, which in turn affects building performance.  A thermal schematic for this process is shown in Figure 3.  Figure 4 shows the application of the weather generator at neighborhood scale in Singapore, where MIT has a major research presence, and Figure 5 shows the modelÕs estimate of changes in urban temperatures relative to a reference in a less-developed area.  Second, as shown in Figure 6, we have produced city-scale temperature maps at 2 pm and 4 am that account for the impact of increased densification.  Figure 7 shows the benefits of potential wide-scale use of such mitigation strategies as green or cool (reflective) roofs, relative to a base case with current roof materials; positive numbers indicate a beneficial reduction in urban temperatures.