The urban outdoor environment presents a valuable addition or alternative to the indoor environment presents a valuable addition or elternative to the indoor environment inhotas well as temperate climates if designed with considerations for human comfort, health and safety. The main airm of the study is to investigate the relationship between urban physical features and human thermal comfort in a warm humid climate, and subsequently to develop easy-touse evaluation tools for deriving climatic design criteria and information towerds the development of urban design guidelines. The emphasis is on longwave radiant energy exchanges. The genaral objective is to generate a frameework of methodologies to evaluate quantitatively the thermal performance of outdoor environment with respect to the relationship between the built forms adand human thermal comfort. To address this general objective the following tasks have been identified and executed. 1) Conducating a field climatic measurement programme of selected urban locations in Kuala Lumpur, using PMV model as the comfort indicator. ii) Determination of a tolerable range of thermal comfort values and related levels of radiant energy exchanges based on PMV model. iii) Estimation of shading effectiveness of urban geometry and orientation using existing computer models (Shawdowpack' 'Obstruct' and 'URBAN3'). iv) Establishment of evaluation tools to predict potential levels of radiant enercy exchanges (modified Stefan-Boltzmann Equation, 'Street and ' Reflect'). v) Evaluation of surface energetics and total radiant energy gluxes within urban outdoor spaces (using 'Obstruct' and 'URBAN3'). The study has confiremed that urban microclimatic conditions in a warm humid climate are sensitive to urban morphology and street layout. It concludes that, contrary to the general views in the literature, there are merits in having deeper urban canyons in warm humid tropics (i.e. buildings located close to each other), in erms of human thermal comfort. Energy budget simulations confirm that shallower urban canyons (H/W ratio of 1 and less) contain substantially more energy fluxes than deeper ueban canyons. Orientation-wise,North/South-oriented streets are most effective compared to other orientations, with NE/SW and NW/SE orientations providing the compromise solutions. It is also been deduced that Building-Height/Street-Width(H/W) ratio of 2 represents the threshold of optimum street shading. Analysis of radiant energy fluxes, based on tolerable PMV levels and commonly occuring local climatic conditions, has identified the thereshold of significant net radiant energy exchanges of 13.5 W/person for a sedentary person. However environmental factor giving rise to net radiant flux differences of over 22.0 W/person unduer similar conditions, must be reviewed and avoided in order to prevent the onset of thermal discomfort due to radiant heat. URBAN3 model has been identified as a potential main evaluation tool for further development work in this area of study, which should include related objectives of models and programs covered in this thesis. The quest for the formulation of urban design guidelines, however, requires comprehensive inputs whicah can address all the basic objectives of designing for the warm humid environment, namely : (a) to minimise solar insolation and urban warmths, and (b) to maximise environmental air flow and dispersion or pollutants. The study has made some contributions towards objective (a).