The Influence of Urban Green Systems on the Urban Heat Island Effect in London

Urban areas are typically warmer than rural ones. This is mainly due to denser configuration dominated by impermeable surfaces such as buildings and roads, compared to rural areas which are less densely built and mainly dominated by open spaces. Rapid urban expansion in dense cities bares direct impact on surface and air temperature patterns within street canyons; a phenomena which is known as the Urban Heat Island (UHI) effect. Thus, several UK city councils such as Birmingham, Manchester, and London have started to develop strategies aiming at enhancing urban green systems (UGS) through trees, green walls and green roofs. Some of those strategies include considering the green space factor, and increasing green areas within the cities to improve street canyon microclimate and reduce UHI. The Mayor of London has adopted a strategy for London 2050 aspiring to transform it to be the greenest city in the world by increasing the green areas up to 50%. This paper investigates the influence of increasing the UGS percentage which is considered as a key solution to mitigate UHI effect which will, in turn, provide thermally comfortable outdoor environments for pedestrians. The investigation is undertaken by comparing the morphology of precincts and streets in relation to air temperature, mean radiant temperature and surface temperature within Oxford Street canyons in London city centre; being one of the world’s busiest streets. The results from this research demonstrate that different UGS interventions with varying percentage are required depending on particular canyon orientations and geometries. The study found that, in general, more trees would have significant thermal comfort effect followed by living façade, while high albedo pavement (HAP) came last. However, HAP had high influence on improving thermal comfort in North-South orientated streets with minor variance to trees and living facades which, changing their percentage levels was insignificant.