Trends in summer air temperature and vapor pressure and their impacts on thermal comfort in China

Abstract

Air temperature and vapor pressure are both considered important factors affecting thermal comfort during summer. However, their variability contributes differently to the comfort level. Based on daily observations over a 56-year period (1961–2016), this study used the Mann–Kendall (MK) test and Theil–Sen’s estimator to examine the trends in summer air temperature and vapor pressure for 648 stations across China. Geographic information system (GIS) grouping analysis was applied to understand the spatial pattern of their changes. Further, based on the assumption of constant wind speed and mean radiant temperature equal to air temperature, the responses of summer daily mean Universal Thermal Comfort Index (UTCI) and the number of summer days with “no thermal stress” to changes in summer air temperature and vapor pressure were investigated. Sensitivity analysis was carried out to identify the areas where UTCI is more sensitive to air temperature and vapor pressure changes. Comparison of the contributions of air temperature and vapor pressure was made to understand the different roles they have played in the changes of UTCI. The results show that approximately 70% of the stations show significant increases in summer air temperature and 33% of the stations show upward trends in summer vapor pressure. As a consequence of the increasing UTCI throughout most of the country, the number of “no thermal stress” days dropped sharply in many areas with an exception of the Tibetan Plateau where this number rose by an average of 0.05 days per year during the study period. Sensitivity analysis shows that summer UTCI is much more sensitive to a unit change of air temperature than that of vapor pressure. However, due to its substantial changes in some regions, including Northwest and Northeast China, the middle and lower reaches of Yangtze River, vapor pressure also played an important role in either intensifying or mitigating the thermal stress in summer.