Cooling with Green Infrastructures: The Influence of Trees on Thermal Conditions in Tropical Urban Parks

Abstract

Heat island effects and their thermal impacts have become a key consideration in the environment of tropical cities. In the face of such conditions, the land use planning and conservation authority of Singapore currently has an on-going island-wide study and plans to manage urban heat at the macro level, but there have been few considerations for the actual thermal environment within these spaces. In tropical climates, solar radiation is the greatest source of heat, and mitigation of solar and terrestrial radiation by landscape design is of utmost importance as compared to other microclimatic factors.

Vegetation and trees in particular have been found to not only intercept direct short-wave solar radiation, but also to reduce reflected short-wave radiation as well as terrestrial radiation in the outdoor environment, through filtering light and radiation with their canopies and providing shade. The micro-scale thermal influence of selected trees was studied during the daytime, comparing characteristics of trees by assessing operative temperature and levels of light infiltration. It was found that canopy density of trees, which determines density of shade, correlates with thermal performance; a tree with a dense canopy can reduce operative temperatures up to 6.3 °C; second, thermal performances ranged widely between surveyed trees. For instance, larger trees remain the most consistent performer throughout the day, even during the hottest time period (12–2 p.m.), where a cool point could be noted. Third, park layouts surveyed demonstrate the critical thermal points of paved area, shaded area, and sun direction.

The thermal performance and cooling function of urban green spaces has known effects in ameliorating not only the spaces themselves but the neighbouring urban blocks too. Studies and findings of this kind have the potential to inform design guidelines and practices, of broad relevance for parks, green belts and other urban spaces, in order to achieve better outdoor environments as well as improved urban microclimates, hence in turn reducing the cooling needs of tropical cities. Whilst the specific design features may be somewhat different, such strategies are equally important for the cities in the hot-dry climates.