Outdoor Thermal Comfort in Urban Environment

Outdoor thermal comfort is determined by urban morphology and the geometry of outdoor urban spaces. The local climate zone (LCZ) classification system aims to characterise the urban and rural land cover based on various urban morphological parameters. It has been widely used in studies of the thermal environment, but the subjective thermal perception between LCZ classes has rarely been studied. This study evaluated the microclimatic conditions and subjective perception of the thermal environment in eight LCZs in Hong Kong, using questionnaire surveys and field measurements. An ANOVA test showed that the microclimatic conditions were significantly different across eight LCZs, and this could be attributed to the urban morphology and the geometry of the outdoor urban spaces. This does not only affect the critical conditions but also the variations in the thermal environment. The highest maximum temperature (38.9 °C) was found in LCZ 1, and the lowest maximum temperature (29.9 °C) was observed in land cover LCZs. Subjective assessment showed that compact or high-rise settings were associated with warmer thermal sensations reported by the respondents. The relationship between the level of thermal stress and subjective thermal sensation changed across LCZs. This study demonstrated that the LCZ classification provides a characterisation of both the physical and thermal environment. It is also one of the first attempts to examine the relationship between the thermal environment and subjective perceptions using the LCZ classification system. Further work is required to investigate how thermal comfort indicators can be used to represent the thermal comfort conditions in different LCZs.

Outdoor thermal comfort studies have proved that urban design has a great influence on pedestrians’ thermal comfort and that its assessment helps one to understand the quality and usage of the pedestrian environment. However, the majority of outdoor thermal comfort studies perceive pedestrian thermal comfort as “static”. The dynamic multiple uses of urban spaces and the highly inhomogeneous urban morphology in high-density cities of the tropics are seldom considered, which leads to a lack of understanding about how pedestrians respond to the changes of the outdoor environment. This study contributes to the understanding of the dynamic thermal comfort using a longitudinal survey that was conducted to obtain information about how thermal sensation changes throughout the walking route and how it is affected by micrometeorological conditions and the urban geometry. The large variations in micrometeorological conditions throughout the walking routes are predominantly influenced by the urban geometry. Additionally, the spatial pattern of thermal sensation varies based on the weather conditions, emphasising the need to account for such variations in the assessment of pedestrian thermal comfort. The results also show that thermal sensation was associated with participants’ short-term thermal experience (2–3 min) and that the urban geometry plays an important role in the time-lag effect of meteorological variables on thermal sensation. The findings of this study contribute to improving urban geometry design in order to mitigate the thermal discomfort and create a better pedestrian environment in high-density cities.

Although outdoor thermal comfort has gained increasing research attention, meteorological conditions and thermal sensation in different urban settings in high-density cities have not been systematically studied from the perspective of urban planning and design. Considering the potential relationship between environmental quality and thermal sensation in outdoor spaces—an emerging topic in perceived comfort, this study offers a new approach for planning and design for climate resilience in cities. This chapter presents the results of an outdoor thermal comfort survey conducted on hot summer days in Hong Kong. Diverse patterns of PET-comfort ratings relationships were found in different urban settings. The study revealed that air temperature, subjective assessments of solar radiation, and wind environment were strong determinants of thermal sensation and evaluation. In our analysis, wind condition showed a significant indirect effect on comfort through subjective perception. Statistical modelling showed that subjective perceptions on microclimate condition and comfort are moderated by various aspects of environmental quality. The findings help inform future design for climate resilience in outdoor urban spaces in hot–humid subtropical cities.

Outdoor thermal comfort has been a widely concerned issue in tropical and sub-tropical cities. In order to assess the conditions of outdoor thermal comfort, quantitative information on different spatial and temporal scales is required. The study in this chapter employs a numerical model to examine the spatial and temporal variations of mean radiant temperature (T_mrt), as an indicator of radiant heat load and outdoor heat stress in high-density sub-tropical urban environment in summer. The SOLWEIG model is found to simulate the six-directional shortwave and longwave radiation fluxes as well as T_mrt very well. Simulation results show that urban geometry plays an important role in intra-urban differences in summer daytime T_mrt. Open areas are generally warmer than surrounding narrow street canyons. Street canyons are sheltered from incoming direct solar radiation by shading of buildings, while open areas are exposed to intense solar radiation, especially along the sunlit walls where high T_mrt is observed due to reflected shortwave radiation and longwave radiation emitted from the sunlit building walls. The present study confirms that there are great potential in using urban geometry to mitigate high radiant heat load and daytime heat stress in the compacted urban environment. In high-density sub-tropical cities where high daytime T_mrt causes severe thermal discomfort in summer, dense urban structures are able to mitigate the extremely high T_mrt and improve outdoor thermal comfort. However, the shading strategy has to be cautious about air ventilation in such a dense urban environment.

Hong Kong suffers from an intense urban heat island effect of up to 4 °C as a result of compact urban form and highly urbanised land cover. Enhancing the cooling efficiency of urban greenery is essential for improving the microclimate in high-density cities. This paper aims to delineate design strategies for urban greenery to maximise thermal benefits and mitigate the daytime UHI effect. Two site-specific design strategies for tree planting in the urban environment are proposed. The sky view factor-based design approach and the wind-path design approach are evaluated in the neighbourhood scale in two climate-sensitive areas with different urban morphologies. Observed data and simulation results indicated that the cooling effect of urban trees is highly associated with SVF. Air temperature reduction (a 1.5 °C reduction) is the most profound for the high-SVF scenario, whereas substantial radiation shading (T_mrt reduced to 34 °C) is detected in areas with medium–low SVFs. The modelling study also showed that the cooling of air temperature and sensible heat were twice as high for vegetation arranged in wind corridors than those for leeward areas. The study demonstrated that tree planting in conjunction with proper planning is an effective measure to mitigate daytime UHI.

Tree planting is one of the veritable tools for combating urban heat island and improving thermal comfort in the wake of global warming and urbanisation. However, trees of different species and morphological properties have variable solar attenuation capacity and consequently, thermal comfort regulation potential. Besides, the shadow-cast effect by buildings helps in reducing pedestrian radiant load and consequently improves thermal comfort, especially in high-density cities even though ventilation is reduced. Therefore, a holistic and contextual understanding of tree planting and shadow-casting can help in designing climate-proof cities. In this study, we employed the ENVI-met model to better understand the interaction between these two forms of shading (trees and buildings) on the pedestrians’ thermal comfort in Hong Kong and the influence of one over the other. The impact of different urban densities on the thermal comfort improvement potential by eight common tree species in Hong Kong was specifically studied. Results show that shallow canyons are susceptible to worse thermal condition when compared to their deeper counterparts with similar aspect ratio value. Of all tree configuration parameters, leaf area index, tree height, and trunk height are most influential in improving and aggravating daytime and night-time thermal comfort, respectively. We also found that trees’ effectiveness in improving daytime thermal comfort reduces with increasing urban density and vice versa for night-time. For the reference of planners and landscape architects, this study recommends tall trees of low canopy density with high trunk in deeper canyons and vice versa for shallow canyons and open areas.

The distinctive features of urban climate have been widely studied for decades. However, the consideration of urban climate in urban planning and design framework is rather limited. One of the possible reasons is due to the difference in working languages between scientists and urban planners such that the urban climatic knowledge cannot be readily utilised in the planning and design practices. The concept of urban climatic map provides an information platform for the presentation of urban climatic phenomena on a two-dimensional spatial map in a format that can be readily interpreted by urban planners. Areas with environmental problems or sensitive to urban climate can therefore be easily identified for mitigation measures. In this paper, the methodology and results of the urban climatic analysis map, as the first part of the urban climatic map study of Hong Kong, are presented and the implications on urban planning and design practices are discussed.