Dynamic effects of frequent step changes in outdoor microclimate environments on thermal sensation and dissatisfaction of pedestrian during summer

Humans engaging in outside activities are more likely to be exposed to frequent spatiotemporal step changes in outdoor thermal conditions, as opposed to constant thermal conditions staying indoors. Understanding pedestrians’ thermal reactions to such dynamic thermal settings is helpful for enhancing outdoor thermal comfort by providing spatiotemporal variations in thermal conditions. In this study, 48 subjects were tested about their thermal perceptions while being exposed alternately to direct sunlight and shade at different defined frequencies, in a series of 45 min experiment period. The experiments were designed to create step changes in microclimate environments. The study was carried out from May to July in a university campus in Hong Kong with subtropical weather conditions. Results show that subjective thermal perceptions varied with alternating exposure to sunlight and shade at different frequencies. UTCI was modified to an equivalent UTCI* for evaluating thermal environments with frequent step changes by taking into account impacts of mixed changes in sun and wind conditions, alternating frequency and expectation on thermal perceptions. With a higher alternating frequency, there was reduced thermal dissatisfaction with hot summer days and a lower comfort requirement for shade, as well as the upper limit of acceptable UTCI* approaching 43.7 °C.