A Review on the Impacts of Urban Heat Islands on Outdoor Thermal Comfort

Abstract

The worsening urban heat island (UHI) effect poses a great challenge to the thermal comfort of people outdoors. However, there has not been a summary of the mechanisms by which UHI affects outdoor thermal comfort (OTC). This paper reviews the commonly used OTC evaluation indexes, data collection methods, and mitigation measures and discusses the relationship between UHI and OTC. The review is limited to peer-reviewed journal publications found in five databases: Science Direct, Scopus, Google Scholar, PubMed, and Web of Science. The review results indicate that physiological equivalent temperature (PET), universal thermal climate index (UTCI), and wet bulb globe temperature (WBGT) are the most widely used indexes in outdoor thermal comfort studies. The data collection methods mainly include questionnaire surveys, measurement, simulation, and formula calculation. There are four main approaches to mitigating the UHI effect in order to improve the comfort of people outdoors: vegetation strategies, water strategies, urban planning strategies, and material strategies. Future research can focus on developing OTC research methods and indexes and combine thermal comfort with visual comfort, auditory comfort, etc. to better evaluate the overall comfort.

Assessment of Outdoor Thermal Comfort in a Hot Summer Region of Europe

“…Atmosphere 2024, 15, x FOR PEER REVIEW 4 of 20 [20,21,25,26], measurements were carried out in different seasons. In this study, experimental measurements were carried out on different days in the summer season in order to obtain the heat stress indices [4] such as UTCI [18,19,21,34,[39][40][41][42][43], PET and mPET [32,33,[40][41][42][43], SET [37,38,40,41,43] and WBGT [34][35][36]41] for the assessment of temporal and cumulative outdoor thermal comfort. Furthermore, the heat stress exposure (HSE) metric [50] was also evaluated using UTCI, as in [50], and using the other indices as well.…”

“…Its main symptom is a high temperature relative to the temperature in surrounding areas [3]. Ren [4] carried out a review where the impacts of UHI that exacerbate this effect are analyzed. One problem to be considered in the UHI is anthropological heat.…”

“…A common method used by researchers to assess thermal comfort is by analyzing the indices of thermal stress experienced by people. In [4], Ren indicated that of the 100 manuscripts reviewed, the most commonly used thermal comfort indices were PET (43), UTCI (14), WBGT (12) and SET (7). Mayer [32] was one of the first researchers to use the PET index to assess thermal comfort in outdoor environments.…”

Study on the Spatial and Temporal Distribution of Thermal Comfort and Its Influencing Factors in Urban Parks

“…Wet-bulb globe temperature (WBGT) is an international standard heat index used by health, industry, sports and climate departments to assess thermal comfort during extreme high temperatures. It is the most widely used heat stress index today [14] and is also considered to be the most suitable index for evaluating outdoor thermal comfort in hot and arid areas [24]. The formula is as follows:…”

“…Among these, more than 165 thermal comfort indexes have been developed for indoor and outdoor applications to quantify the thermal environment, but only a few are in use [12]. Standard effective temperature (SET) [13], wet-bulb globe temperature (WBGT) [14], heat index (HI) [15], physiologically equivalent temperature (PET) [7], PMV-PPD [16], thermal discomfort (DISC), etc., are all commonly used thermal environment evaluation indexes, but there is no conclusion as to which is the best index.…”