From Thermal Sensation to Thermal Effect: A Multidimensional Semantic Space to Assess Outdoor Thermal Comfort

Outdoor thermal comfort is critical to citizen’s health and well-being as well as recreational and commercial activities in urban spaces. Previous research has widely adopted psychometric tool for outdoor thermal comfort assessments, such as 7-point thermal sensation scale, which was originally developed for indoor thermal comfort research. In such definitions, the thermal neutral sensation is linked to the most comfortable thermal status. However, particularly in outdoor spaces, this scale fails to adequately describe outdoor thermal comfort status mainly due to 1) high spatial and temporal variabilities in wind speed and solar radiation outdoors compared with indoors, and 2) the sensation scale’s omission of affective information (e.g. pleasant or unpleasant).

To address these shortcomings, this study aims to develop a multi-dimensional thermal affect semantic space for outdoor thermal comfort assessments that includes both descriptive and affective information. ‘Affect’ is a psychological term used to represent the experience of feeling or emotion evoked by certain stimuli. By extension, outdoor thermal affect means the feeling or emotion elicited by outdoor thermal stimuli. To obtain this objective, a combination of online questionnaires and field experiments are used, summarized in Fig. 1. First, an online questionnaire containing semantic differentials was employed to collect the thermal perception of 76 quotidian thermal adjectives on four descriptive dimensions - temperature, humidity, wind speed, solar radiation - and two affective dimensions - thermal pleasure and thermal intensity (Figure 1). Second, a field experiment was conducted in Sydney to collect subjects’ real-time thermal perceptions on six dimensions used in the online questionnaire, and the most representative adjectives of their thermal feelings in actual outdoor thermal environments. The scores of the most commonly used adjectives under two different experimental conditions – online questionnaire and field assessment - were compared to validate the semantic space of outdoor thermal affect.

When comparing the online questionnaires and field experiment results, we find that scores of the thermal affect words showed good consistency on temperature and thermal intensity dimensions, and none of these words changed its valence – from unpleasant to pleasant or vice versa – between two experimental conditions. This result revealed that the semantic interpretation of these climatic adjectives can reflect the subjective thermal feelings in terms of the temperature, thermal intensity and the thermal pleasure valence. On the humidity scale, words close to the dry extreme agree well in two experimental conditions while near the humid side, the field assessments were often less extreme than on the online questionnaires (Figure 2). A possible reason is because of the relatively mild and moderately humid condition of Sydney during the field survey periods. On the radiation scale, instead of distributing continuously along the solar radiation scale as the online questionnaires, two obvious clusters were observed near the two end points of the solar radiation scale in field assessments. This suggests that there is a turning point for the actual solar radiation perception: people sense either mild solar radiation below that point or strong radiation beyond the threshold. In addition, both online questionnaires and field assessments revealed that one thermal sensation scale is semantically inadequate to capture the lived outdoor thermal comfort experience. Moreover, neutral thermal sensation doesn’t necessarily reflect the most comfortable state in urban outdoor climates (Figure 3).

The study initiates a fundamental shift from focusing on simplistic one-dimensional descriptive thermal sensation scale towards a multi – dimensional thermal affect semantic space in outdoor thermal comfort assessments and provides an approach to evaluate the outdoor thermal affect. Instead of the primitive and bland descriptors or numbers used in previous thermal comfort research, we think language is a more effective and evocative way to communicate with others, and its meaning is adequately rich to capture the whole experience of the vivid urban outdoor climate.