Evaluation of microclimates and thermal perceptions in urban precincts

Abstract

The fast pace of urbanisation has led to more built up spaces in many urbanised areas. Urbanised areas influence surrounding microclimates, which in turn, affect users’ thermal perceptions, well- being and outdoor usage patterns. Thermal comfort, which has been extensively studied and used for indoor spaces such as offices and residential buildings, is one of the measures used to assess outdoor spaces. In the absence of outdoor thermal comfort standards, the researchers studying thermal comfort in outdoor urban areas have begun to take advantage of the standards developed for indoor conditions (ISO 7730 2006, ASHRAE 55 2010) to assess outdoor thermal perceptions. However, there are some debates about the adequacy of such standards in various contexts including indoor and outdoor conditions. Some thermal comfort literature has emphasized on the necessity for revising the ‘philosophy’ that forms the comfort standards, which was the stance of the adaptive approach to thermal comfort. In contrast to heat balance theories, the adaptive thermal comfort model includes some influential contextual factors in the assessment of thermal comfort. Australia and particularly Melbourne, capital city of Victoria, is one of the world’s major education providers. Each year many students are admitted to Australian universities and the number of local and international students is expected to rise in the future. The resultant urbanisation in Australia’s cities has created modified meteorological conditions affecting people’s thermal comfort. A university campus attended by people from different climatic backgrounds represents an environment with varying thermal expectations and preferences, providing a great opportunity to investigate the extent of influence of contextual factors on people’s thermal perceptions and applicability of the existing standards. Consequently, this study was carried out at the RMIT University City Campus, an educational urban precinct located in the heart of Melbourne’s central business district (CBD). This study developed a research hypothesis, “existing thermal comfort standards are not adequate to assess the determinants of outdoor thermal comfort conditions”, to investigate the applicability of the assumptions enshrined in the thermal comfort standards in the context of educational urban precincts in Melbourne. Accordingly, three research questions were formulated to navigate the research: (1) to what extent are the thermal comfort standards applicable to educational urban precincts in Australian cities? (2) to what extent can contextual factors influence outdoor users’ thermal perceptions? and (3) what are the factors influencing usage patterns and behaviours in educational outdoor spaces? To investigate people’s interaction with thermal conditions in outdoor built environments, three rounds of field surveys (spring 2014, summer 2015, and autumn 2015) were conducted. Field surveys consisted of questionnaire surveys and concurrent measurements of four environmental parameters that are best known to have the most impact on people’s thermal subjective assessment: air temperature (Ta), relative humidity (RH), wind speed (Va) and radiant temperature (Tg). The questionnaire was structured according to ISO 7730, ASHRAE 55, and ISO 10551 and aimed to capture people’s thermal perceptions (including thermal sensation, preference, acceptability, and overall comfort). Three thermal comfort indices, namely Physiological Equivalent Temperature (PET), Outdoor Thermal Climate Index (UTCI) and Outdoor Standard Effective Temperature (OUT-SET*) were employed to predict thermal comfort conditions using the four thermal factors and two personal parameters (level of activity and clothing insulation). In total, 1059 questionnaires were collected from the three sites of RUCC. The findings on usage pattern of study sites showed that “time of the day” and “weather conditions” were the two major determinants of people’s outdoor attendance. The results revealed that the main assumptions regarding the orthodoxy of thermal comfort (thermal neutrality/neutral temperature1 and acceptable thermal range2) being based on thermal sensation scale was not applicable to the context of an education precinct in Melbourne. Instead, the derivative of thermal preference scale (preferred temperature3) was found to be a better representative of people’s thermal satisfaction and thus thermal acceptance. Therefore, a multi-model research framework was developed to understand the discrepancy between the patterns of observed comfort data and recommendations enshrined in standards regarding thermal satisfaction. This framework consisted of “Socio-ecological system model (SESM)”, “theory of Alliesthesia” and “theory of rising expectations”. The modified version of SESM in this study assumes that several contextual factors clustered under five environments (individual, social, physical, psychological, and standards and guidelines) influence people’s thermal sensations. The results obtained from the analyses of SESM environments suggested that in total, 12 out of 29 contextspecific factors were identified as having a medium impact on people’s thermal sensations. The findings are in line with the notion of adaptive comfort theory according to which non-thermal factors can influence people’s thermal expectations, preferences, and thus their thermal satisfaction. The psychological concept of “Alliesthesia” was used to explain the noticeable variations found in the people’s preferred temperature in different seasons. This concept refers to the notion of “thermal pleasure” whereby people prefer an opposite thermal status once they have had enough experience of current thermal conditions, since repeated exposure diminishes its desirability over time. In other words, people perceive a warm or cold stimulus to be pleasant or unpleasant when their body core temperature is above or under normal conditions. In winter, people yearn for the warmer conditions of the summer months, while in the heat of summer, they yearn for cooler winter conditions. The last component of this framework, “rising expectations”, justified higher thermal expectations of people interviewed in this study (local and international students studying in an Australian university) by referring to their tendency to set greater life standards including higher thermal expectations. According to this theory, when there are some improvements in people’s quality of life, they tend to get used to it and even raise it; dissatisfaction occurs when there is a failure in constant provision of such ideal conditions. Highlighting the inadequacy of current thermal comfort standards, this study attempted to indicate the need for revisiting such standards whereby the results of comfort assessments will be better representative of thermal comfort requirements in real world conditions. The accurate definition of thermal comfort requirements will provide a platform to improve outdoor thermal conditions and advance other related disciplines, including but not limited to, urban design, planning, urban meteorology, and health and safety