A quantitative assessment of the dependence of outdoor thermal-stresses on tree-building morphology and wind: A case-study in sub-tropical Patna, India

Abstract

This study quantifies the changes in thermal-stresses due to changes in tree-building-morphology and background-wind, at a site in sub-tropical Patna, India, where new residential buildings are under-construction, after demolishing the old, including felling of around 180 trees. Six morphological-variants are compared through simulations, using the diagnostic model SkyHelios Pro, to identify the individual/synergetic thermal-effects of different morphological-attributes/ background-wind. SkyHelios allows for a fast spatial/temporal analysis of the thermal changes at a point of time/space, giving maximum output parameters. The highest spatial-mean-rise in mean radiant temperature/ physiological equivalent temperature of 3.4/2.9 °C occurs at noon, at background-wind 4.0 m/s, due to trees-removal-area of 30 % and built-up area addition of 79 %, simultaneously. Individual changes manifested by trees and buildings are quantitatively conservative/additive. Buildings contribute heat, related positively to horizontal built-up density, negatively to wind-permeability, and regardless of building-height/volume. Spatial-mean cooling by trees is higher at lower-winds, related positively to plantation-density though not necessarily linearly, depends on tree-building overlaps, ground-conditions, and wind-permeability. Wind-sheltered zones, oblique-narrow canyons, and building-skins are dominant heat-contributors and best beneficiaries of tree-shading at all background-winds. The study is limited to day-hours, excludes thermal-effects of tree-species and building-materials. Recommendations for passively-cooled outdoors, a core-issue concerning thermally-sustainable cities, are proposed.