Urban human thermal comfort in hot and humid Hong Kong
Highlights
► Air temperature, wind speed and solar radiation influence outdoor thermal comfort. ► The neutral physiological equivalent temperature in summer Hong Kong is about 28 °C. ► 0.9–1.3 m/s wind is needed for neutral thermal sensation under shaded condition.
Introduction
Outdoor meteorological conditions have profound influences on the comfort sensation of people and therefore are important factors to be considered in the design and planning of urban outdoor spaces. Numerous researches have been done [1] especially in outdoor spaces for developing appropriate thermal indexes [2], [3]. Nikolopoulou et al. have conducted studies across Europe to understand the human parameters in outdoor urban spaces [4], [5]. In the tropics, the science and physiological factors of discomfort has been investigated in general by Gagge and Gonzalez [6], and by Admad [7]. Case studies in Hong Kong by Cheng and Ng [8], in Taiwan by Hwang and Lin [9], in Japan by Givoni et al. [10] and by Thorsson et al. [11] and in Australia by Spagnolo and de Dear [12], have respectively studied the comfort zone of outdoor spaces in hot-humid environment. Ali-Toudert and Mayer furthered this with a study on street characteristics on thermal comfort [13]. They have provided understanding of thermal sensation of people in different outdoor spaces; under different climatic conditions; and with different adaptive behaviours.
For Hong Kong, Cheng and Ng [8] collated data of earlier researchers conducting studies elsewhere that are of similar climatic conditions of Hong Kong constructed the Hong Kong outdoor comfort chart [6], [14]. The chart shows that for a person under shade (solar radiation intensity of around 100 W/m2) and in Hong Kong average summer air temperature of 28 °C, one needs around 1–1.5 m/s air speed to remain in comfort (Fig. 1).
Outdoor thermal comfort studies are typically conducted by means of transverse questionnaire surveys, where large numbers of subjects were interviewed in various environmental conditions. Based on this method, the responses gathered can provide a statistical estimated thermal sensation of an average person under static climatic conditions. There are limitations arise from this method. Since thermal sensation was captured under a relatively static climatic condition during the survey, the transverse approach cannot reflect the effect of changing climatic conditions, for example increased solar exposure, on thermal sensation [15]. Nonetheless, to gain a general statistical understanding of the outdoor thermal comfort condition in Hong Kong, transverse questionnaire surveys are suffice for this study.
Section snippets
Methodology
The transverse thermal comfort survey employed in this study has an international standing in the field of outdoor thermal comfort research; it has been widely adopted in outdoor thermal comfort studies all over the world ranging from tropical [1] to temperate [16] and continental climates [17], [18], [19]. The EU-funded RUROS project has employed this method to understand the outdoor thermal comfort conditions in seven cities across five countries in Europe [8], [9], [20]. de Dear has applied
Results of the study
In Hong Kong, majority of the indoor spaces are cooled in summer. We suspected that subjects who have and have not been in an air-conditioned space prior to the survey may have different thermal responses due to the effect of acclimatization. Acclimatization refers to changes in the settings of a person's physiological thermoregulation system over a period of time, in response to exposure to single or a combination of thermal environmental stressors [3]. In order to take into account the effect
Neutral PET and the need for wind
Since PET is a function of climatic variables [36], the survey data can be further analysed by resolving the PET understanding based on the constituting climatic conditions. The aim of this analysis is to establish the wind requirement for Hong Kong people using the mean nPET.
The foregoing analysis has established a summer mean nPET of 28.1 °C for an outdoor air temperature of 27.9 °C. This finding is consistent with the survey results which indicate a range of neutral PET of 27–29 °C. For nPET =
Discussion
The summer months are considered to be of particular concern for urban thermal comfort in Hong Kong. The Hong Kong Observatory long-term air temperature data have been used to establish the neutral PET (nPET) under typical Hong Kong summer condition. Based on our analysis, it is established that the summer mean nPET is 28.1 °C, and around 50% of the survey subjects express neutral thermal sensation when PET is in the range of 27–29 °C, followed by 32% “slightly warm”, 13% “hot” and 4% “very hot”.
Conclusion
All in all, based on the survey findings, it can be recommended that there is a need:
- 1.
To improve Hong Kong wind environment by ensuring a conducive wind environment with wind speed of 0.9–1.30 m/s in the city through better planning, optimal development potentials and building coverage, layout and disposition.
- 2.
To reduce solar radiation gains on pedestrian in streets or in urban open spaces by providing shades. Canopies covered building recesses and walkways, colonnades, and so on are useful. Tmrt
Acknowledgement
The authors thank the Planning Department, HKSAR Government for funding the project. We would like to thank Mr. S.C. Lau of Planning Department and Professor Y.K. Chan of CUHK for assisting with the statistical analysis of the survey data. Many student interviewers and research assistants, especially Ms Cecilia Chan, have helped with the field work. Technician Max Lee has helped with the equipments.
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