Thermal comfort implications of urbanization in a warm-humid city: the Colombo Metropolitan Region (CMR), Sri Lanka

In this paper we analyze the historic trends in thermal comfort (measured in terms of Temperature–Humidity Index [THI] and Relative Strain Index [RSI]) in the Sri Lankan primate city of Colombo and correlate them with land cover changes in the region. Land cover is calculated form time-series aerial photographs in terms of “hard” cover (buildings, paved areas and roads) and “soft” cover (trees, green areas and waterbodies). The period selected for analysis includes pre-rapid (up to 1977) and rapid urban phases (1978 onwards) in the city. Contemporary Sri Lankaʹs urbanization is peculiar in that mid to late 20th century urban rates (approx. 22% of the population) had remained virtually unchanged till the economy was liberalized in 1977, but have recently intensified (currently at about 35%). This offers a unique window of opportunity to look at the thermal comfort transition consequent to urbanization. Since many tropical cities are at a similar stage of demographic transition, lessons from Colombo may generally be applicable to other tropical developing cities as well. An increasing trend in thermal discomfort—particularly at night—is seen especially at the suburban station and it correlates well with hard land cover changes. The study also brings out the relative importance of land cover in city center vs. rural areas (e.g. hard cover has more effect on thermal discomfort in city center than in rural areas). Based on these findings, we postulate an outline for a climate-sensitive urban design policy for tropical cities.