Abstract
Emerging tropical cities are experiencing rapid population growth and development, which can greatly affect the thermal environments. The effects of roadside trees and road orientation on the outdoor thermal environment were investigated on four different roads in Kuala Lumpur, Malaysia. Field measurements were conducted to assess outdoor thermal environments, where the selection of sites was based on different roadside tree morphological features and road orientations. Outdoor air temperature (Ta), relative humidity (RH), globe temperature (Tg), wind speed (WS), and wind direction (WD) were measured. Absolute humidity (AH) was estimated based on relative humidity and air temperature. Planting dense canopy trees with an average sky view factor (SVF) of 0.07 reduced the mean radiant temperature (Tmrt) by 35% and the physiological equivalent temperature (PET) by 25%. East–West (E–W) and Northwest–Southeast (NW–SE) oriented roads had high PET values of 41 °C and 43 °C, respectively. North–South (N–S) and Northeast–Southwest (NE–SW) orientated roads had lower PET values (37 °C), providing improved outdoor microclimate. Roadside trees provided greater cooling potential in E–W and NW–SE oriented roads. The findings are useful for urban road design in tropical cities in order to improve the outdoor thermal environment and pedestrian comfort.
Highlights
Climate change has affected human health through heat-related illnesses, which will continue based on future predictions [1,2]
Our results indicate that the density of roadside trees affected Tg, which in turn affected the outdoor thermal environment
Mitigation effects of roadside trees on outdoor meteorological parameters were shown, with Ta, Tg, and Ts values decreasing on average by 4%, 16%, and 21%, respectively, for dense roadside tree canopies comparing to the reference location
Summary
Climate change has affected human health through heat-related illnesses, which will continue based on future predictions [1,2] This includes an increase in the frequency and intensity of heat waves and thermal stress, which can result in an increase in human mortality rates [1]. In addition to climate change, outdoor air temperatures in urban areas have increased significantly due to rapid development and contribution from anthropogenic heat, such as heat release from automobiles, air conditioning, and industrial facilities. These factors have resulted in urban areas having higher temperatures than their rural counterparts, known as the urban heat island (UHI) phenomenon [5,6]. The UHI effect and the outdoor thermal environment have been studied extensively worldwide [9,10,11,12]
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