SEMANTICS OF THERMAL COMFORT IN URBAN OPEN SPACES

Abstract

Cities around the world need to adopt a ‘climate-responsive’ approach to urban open space design as they grapple with mounting heat stress. The provision of comfort outdoors is consequently gaining enhanced importance in urban planning and design. Thermally comfortable open spaces promote conviviality; wellbeing and health among city dwellers. It is imperative to note however that outdoor environments are complex. They are thermally and spatially diverse and are characterized by varying environmental stimuli. In order to design for comfort outdoors, a comprehensive understanding of the micrometeorological conditions prevalent in outdoor spaces and associated user perception is vital. Comfort studies have witnessed a shift in approach from being ‘purely physiological’ to the ‘adaptive’. Interestingly, researchers have also begun engaging with the perception of thermal processes; thereby highlighting fresh perspectives with respect to users’ thermal experience in open spaces. Consequently, comfort research has witnessed the emergence of renewed interest in studies that explore the intersection of biometeorology with environmental psychology. This understanding is particularly useful for designers. For, designers conceptualize outdoor spaces which are eventually experienced by users as ‘thermal places’ with varied meanings. Although many studies that have attempted to address the issue of thermal stress and the occurrence of heat islands in Indian cities; studies focusing at the micro – scale are relatively scarce. Also, limited work has been carried out from a micrometeorological and thermal standpoint, within the scope of public space design in Indian cities. It is paradoxical to note that urban thermal comfort studies are especially lacking in the Indian context. Studies at the micro- scale can inform the evidence based approach for design of outdoor spaces and are therefore crucial. In view of the above, this study deals with the outdoor thermal comfort assessment in three urban open space typologies and hypothesizes that comfort is context sensitive. Micrometeorological monitoring and concurrent transverse comfort surveys were conducted during both summer and winter in Chandigarh, eliciting 2523 valid responses. The relationship between mean radiant temperature and radiant flux densities is also studied. Based on the data collated, crucial outdoor thermal benchmarks are defined for the urban open space typologies in the city. Thermal neutrality for all survey respondents (n = 2523) across all seasons and open space typologies corresponds to a PET value of 24.09 °C. Thermo- neutrality benchmarks varied seasonally. Thermal sensitivity is higher during summer with unit change in thermal sensation vote being effected through a 7.01 °C change in PET values as compared to 9.27 °C change in PET value during winter. The acceptable PET range determined in this study is 20.27 – 36.64 °C (R2 = 0.80). Thermal sensitivity varies based on open space typology as well. The acceptable thermal range is the widest in the green (9.53 – 44.79°C PET) and narrowest in the Plaza (25.43 – 34.50°C PET). The outdoor thermal comfort model has been developed based on different statistical techniques. The crucial micro-meteorological determinants of thermal sensation are determined by step-wise regression. Air temperature (Ta) is the most important variable affecting thermal sensation in urban open spaces of Chandigarh. An increase in wind speed and reduction in radiation correspond to an increase in the neutral air temperature. A unit increment in mean TSV is effected by an increment of 6.63 oC in Ta values in the sun exposed conditions. In contrast, a unit increment in the mean thermal sensation value is effected by an increment of 7.67 oC in Ta under shade conditions. The optimum wind speed which respondents consider “appropriate” is 1.25 ms-1 (R2 = 0.83). However, micrometeorological parameters alone cannot explain the thermal perception of users outdoors. Design attributes play a decisive role in shaping the thermal environment; thermal perception and comfort assessment outdoors. The influence of design attributes; non – thermal factors and environmental stimulation on thermal perception and adaptation is emphasized in the second part of the study. Additional studies conducted in the designed open air markets - Haats in New Delhi and socio- cultural hub Gurugram underscore how choice of material; landscape elements; adaptive opportunities etc. shape the micrometeorological conditions and users’ thermal perception. These studies also helped establish the crucial determinants of thermal comfort in designed squares. For example, the design descriptor pertaining to sky view is a crucial determinant of PET and Tmrt. Study of radiation profile of various landscape surfaces helps in developing an understanding of the thermal behavior of surfaces and their application in outdoor spaces. Interestingly, environmental cues in terms of proportion of space, material and color palette and satisfaction with landscape elements provided; influence comfort perception outdoors. Non thermal attributes such as purpose of visit, gender and kinetic state of respondents play a decisive role in influencing thermal sensation and acceptability. Shade provision (e.g. trees and canopies); cool pavements and misting systems successfully ameliorate the micrometeorological conditions prevalent. Finally, although the findings of this study cannot be applied universally, they can be used as a means of comparative studies. This study should be viewed as an attempt to evaluate open spaces from a thermal comfort standpoint; establish thermal benchmarks and suggest recommendations for designing thermally comfortable open spaces.