Abstract: Using the observational data of Shijiazhuang meteorological station and township stations (Shenzhou and Jinzhou) at the same latitude from 1957 to 2010, the inter-decadal variations of sultry days and sultriness indices and their differences between Shijiazhuang and township station were analyzed. In addition, the inter-decadal variations of temperature, relative humidity and absolute humidity at 14:00 BT in summer and sultry weather were also analyzed. The results show that during 1957-2010, sultry days in Shijiazhuang increased gradually, while the township station showed bimodal distribution. The variation of sultry days at Shijiazhuang was consistent with the variation of temperature at 14:00, which indicates that temperature plays a leading role in enhancing sultry level. The sultry days at Shenzhou were related to the temperature and humidity at 14:00, with major role in humidity. Under urbanization, urban heat island effect at Shijiazhuang was getting stronger than that at Shenzhou from the late 1980s, and the dry island effect was enhanced after the middle 1990s. The sultry days in Shijiazhuang were mainly affected by the urban heat island effect and global warming, while the drop of relative humidity due to the urban dry island effect had an adverse impact on the formation of sultry weather, which reduced part of the possibility of sultry weather.

Key words: sultriness index, urban dry island effect, urbanization