INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2017) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.5038 Changes of effective temperature and cold/hot days in late decades over China based on a high resolution gridded observation dataset Jia Wu, a Xuejie Gao, b,c * Filippo Giorgi d and Deliang Chen e a National Climate Center, China Meteorological Administration, Beijing, China b Climate Change Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China c University of Chinese Academy of Sciences, Beijing, China d The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy e Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Sweden ABSTRACT: The effective temperature (ET) is employed to investigate observed changes of thermal comfort conditions over China during the late decades of the historical observational period. ET considers the aggregate effects of temperature, relative humidity, and wind speed to describe the human thermal sensitivity to weather and climate. The data used in the study is the recently produced gridded daily scale dataset CN05.1, which covers the period of 1961 – 2014 at a resolution of 0.25 ∘ latitude by 0.25 ∘ longitude. Results show a general increase of ET in both December-January-February (DJF) and June-July-August (JJA). The increase is mostly caused by an increase of temperature and a decrease of wind speed, while the contribution from relative humidity is small. As measured by ET, China is a cold country with many more cold days than hot days. Large decreases of cold days and increases of hot days are found following the increase of ET. The number of comfortable days shows larger values in the warm areas and during the warm seasons. During the analysis period, the number of annual comfortable days shows an increasing trend when considering the China-wide average. However, the number of annual comfortable days during JJA decreases over warm areas. KEY WORDS climate change; CN05.1 observation dataset; effective temperature; China Received 28 August 2016; Revised 21 January 2017; Accepted 23 January 2017 1. Introduction Global warming greatly affects our living environment by changing frequency of thermal sub-comfort and dis- comfort (both cold and hot), and in fact it can endanger the habitability of increasingly wider areas (e.g. Pal and Eltahir, 2015). Therefore, it is important to explore the change of thermal comfort conditions in the context of global warming. While human comfort/discomfort, morbidity and mortality depend largely on temperature, other climate variables such as speciic humidity, wind speed, and solar radiation are also signiicant factors (e.g. Epstein and Moran, 2006; Blazejczyk et al., 2012). Various biome- teorological indices have been used to assess the human perception of temperature, or the thermal comfort, and the resulting thermal stress. They are based on the combina- tion of temperature, humidity, wind speed, and possibly other variables, as summarized for example by Epstein and Moran (2006), Blazejczyk et al. (2012), and Freitas * Correspondence to: X. Gao, Climate Change Research Center, Insti- tute of Atmospheric Physics, Chinese Academy of Sciences, No. 40 Yanhuali, Chaoyang District, Beijing 100029, China. E-mail: gaoxue- jie@mail.iap.ac.cn and Grigorieva (2015). Some of the relatively simpler indices have been used in analysing changes of heat waves under global warming conditions (e.g. Diffenbaugh et al., 2007; Fischer et al., 2012). There have been several studies on the impact of weather and climate conditions on human comfort through the application of comfort indices for China; however, most of these studies were conducted at local scales and focused on the tourism sector (e.g. Fan and Guo, 1998; Ren et al., 2004; Wang et al., 2012b; Yan et al., 2013; Zeng and Dong, 2015). Conversely, stud- ies covering the whole mainland China have not been carried out. The effect of relative humidity on human comfort under mild temperature conditions is not critical. However, under hot conditions such as during the Chinese summer, high humidity reduces sweat evaporation from the human body needed for cooling and consequently increases heat stress. Wind removes heat by accelerating evaporation in hot conditions, and by convection in cold conditions, thereby leading to a chilling effect. This latter effect is especially pronounced in northern China during periods when strong northerly winds blow cold air from Siberia, often every 1 or 2 weeks. As of the greater heat conduction of humid air, the wetter climate of southern China (SC) in winter © 2017 Royal Meteorological Society