Comparisons of Meteorological Conditions on Fog, Haze, and Clear Days in Anhui Province, China

Based on differences in meteorological conditions on fog, haze, and clear days, the authors concluded that the forecasting methods and index of fog and haze should be set up for different regions based on local conditions. Among the surface meteorological parameters, visibility and relative humidity showed the most evident differences in the above-mentioned three weather conditions. According to the visibility and relative humidity at 2000 LST in the preceding day and 1400 LST on the current day, all studied sites in Anhui Province were divided into three categories:1) From fog, haze to clear days, the visibility increased and relative humidity decreased evidently (e.g. Hefei); 2) Visibility and relative humidity both were very close on fog and haze days, but were evidently different from those on clear days (e.g. Fuyang); 3) Visibility showed insignificant differences between fog and haze days, while relative humidity showed evident differences between fog and haze days (e.g. Anqing). More than 50% of fogs were followed by haze at urban sites; while less than 25% of fogs were followed by haze at countryside sites. In the vertical, the relative humidity decreased rapidly with height during fog days, with the median value lower than 20% at 850 hPa at Anqing station and lower than 45% at Fuyang station; however, it decreased slowly during haze days with the median value of around 60% at 850 hPa at both stations. In addition, the wind shear from the middle to upper boundary layer was smaller on haze days than on fog and clear days.