Discussion on inapplicability of Universal Thermal Climate Index (UTCI) for outdoor thermal comfort in cold region

The Universal Thermal Climate Index (UTCI) is one of the commonly used thermal indices to evaluate outdoor thermal comfort. In order to explore its applicability in cold regions, UTCI was compared with the Physiological Equivalent Temperature (PET) and outdoor air temperature (Ta) simultaneously. And the internal mechanism relationship between UTCI and outdoor environmental parameters were explored. The results showed that UTCI, PET, Ta had a good linear relationship with outdoor human thermal sensation. The probability of correct interpretation for outdoor thermal comfort was 74.2%, 75.1%, 80.1%, respectively. UTCI may have an invalid interval under its low-temperature zone in cold region. With the decrease of UTCI in the invalid interval will not affect the subjective thermal sensation and objective average skin temperature significantly. Taking Tianjin for example, the invalid interval of UTCI was approximately lower than −15 °C, which may be due to UTCI being very sensitive to the outdoor lower air temperature (approximately lower than 0 °C) and higher wind speed (approximately above 8 m/s). This study does not recommend UTCI as an evaluation index for outdoor thermal comfort in cold and severe cold regions in winter.