Surface Fraction as a variable for Urban Heat Island Amelioration in Colombo N G R Perera, B L T Langappuli Department of Architecture, University of Moratuwa, Sri Lanka nareinperera@gmail.com Abstract Rapid urbanization has resulted in the change of land use and thus land cover from rural natural, pervious green surfaces to impervious urban land. It is identified as a key reason for microclimatic changes that create the Urban Heat Island (UHI) phenomenon that effect many cities. Although, the planning parameters for Colombo, Sri Lanka do specify the building to non-built surface fraction, it does not define the nature of the non-built areas. It is also deemed that such planning and building regulations are not based on any overall climatic goals for the city. In this context, the need is to quantify urban parameters that can be controlled by urban design. This study explores the effect of the building surface fraction, impervious surface fraction, and pervious surface fraction of an urban block, as a strategy for UHI amelioration in Colombo. The range of the combinations of the above are limited to the range defined by the Local Climate Zone (LCZ) classification system. It is further focussed on the predominant LCZs of Colombo, LCZ3 - Compact Lowrise and LCZ2 – Compact Midrise. The study uses the computer simulation software ENVI-met to model the existing as well as the modelled surface fraction of a particular urban block, in Colombo. Results and Analysis will discuss the comparative implications of the changing surface cover on the UHI mitigation possibility in warm humid Colombo. Keywords –Surface fraction, Urban Heat Island, Warm Humid Tropics, Local Climate Zone, Sri Lanka Introduction The rapid urbanization has brought in its wake to many challenges to Sri Lanka's largest commercial city. One such significant challenge is the warming of the city and therefore the increased energy use. This is evident by the well documented Urban Heat Island (UHI) phenomenon that affects the city. Urbanization has resulted in change of land use / land cover from natural green surfaces to impervious urban land. It is identified as a key reason for microclimatic changes that create the Urban Heat Island (UHI) phenomenon. The study explores the