Street canyon geometry and microclimate : Designing for urban comfort under arid conditions

While urban design guidelines have been developed for responding to climate in various regions, these recommendations are often based more on intuition or sporadic observation than on an integrated microclimatic analysis of thermal comfort conditions. Quantitative studies on desert environments are especially lacking, since most arid regions remain sparsely populated. In the present study, empirical data taken from extensive full-scale measurements in a number of low-rise urban street canyons in the arid Negev region of Israel are integrated with a simple numerical model representing the overall thermal energy exchange between a pedestrian and the street canyon environment. The integrated thermal index produced allows a comprehensive means for comparing geometric alternatives and generating guidelines which can aid in the design of urban spaces under climatically similar conditions. Analysis of overall energy balance suggests that in summer, overheating within the canyon is sensed primarily as a nocturnal phenomenon, and that during hours of substantial heat stress in a desert climate, compact urban spaces do in fact constitute potential “cool islands”, mainly dur to internal shading. In winter, a compact geometry was found to provide relatively warm conditions during most hours, with the key factor being protection from chilling by strong winds.