Recent urban microclimate studies in Colombo, Sri Lanka, indicate that the maximum daily temperature within street canyons decreases with increasing height to width (H/W) ratio, but higher H/W ratio negatively affects street-level wind flow. There is also evidence pointing to the cooling effect of sea breeze. The nocturnal heat island is small in contrast to daytime urban-rural differences. In this paper, we use the software ENVI-met to simulate the effect of different urban design options on air and surface temperatures, as well as on outdoor thermal comfort. The latter is expressed as the physiologically equivalent temperature (PET), an index based on air and radiant temperatures as well as wind and humidity. It is found that high albedo at street level gives the lowest air temperature during daytime, although the reduction is only about 1 degrees C. The lowest daytime mean radiant temperatures result from high H/W ratios of streets. This has a positive effect on thermal comfort; the increase of H/W ratio from about 1 to 3 leads to a decrease in PET by about 10 degrees C. Differences in air and surface temperatures, as well as PET, are small during the night. The results show that strategies that lead to better air temperature mitigation may not necessarily lead to better thermal comfort. However, shade enhancement through increased H/W ratios is clearly capable of significant reductions in PET, and thus, improved outdoor thermal comfort. Consequently, a critical urban design task in the humid tropics will be to guide the rapid urban growth towards efficient 'shade growth'. Copyright (c) 2007 Royal Meteorological Society.
