Analysis on the temporal change features and meteorological conditions of fog day in Jiuhua Mountain

The conventional observation data and the intensive observation of automatic weather station from 1991 to 2010 in different altitude of Jiuhua Mountain have been employed to analyze the temporal change features and meteorological conditions of fog day in the mountain area. The results show that the interannual change of fog day on the mountainside is bigger and decreases yearly but fog day on the foot of mountain increases yearly in the latest 10 years. The mean fog day (8 d) in autumn and mean fog day (7 d) in winter is more than the one (3 d) in spring and one (2 d) in summer on the foot ofmountain, but the mean fog day (50 d) in winter and mean fog day (45 d) in spring is more than the one (27 d) in summer and one (26 d) in autumn on the mountainside. The fog occurs easily from 5 a.m.to 7 a.m. and disappears mainly from 8 a.m.to 10 a.m. on the foot of mountain, but the fog occurs easily from 4 a.m.to 8 a.m. and disappears mainly from 9 a.m.to 11 a.m. on the mountainside, and there is a difference in the fog duration between mountainside and the foot of mountain. The fog day accounts for 74.4% of the total when temperature in the nighttime fall more than 6 ℃; the fog day accounts for 80.9% of the total when diurnal range of air temperature is more than 7.0 ℃. 65.2% of the total fog days have the feature that mean relative humidity is over 90% from 20 p.m. in a day to 08 a.m. on the following day. Most of fog days occur when wind speed is less than 3 m·s^-1, and 83.9% of the total fog days occur when an inversion layer exists near the ground. The change of humidity condition is favorable to originate fog in the trumpet-shaped topography areas after unsaturated wet air gets into the terrain; meanwhile, the convergence of wind field in the mountain area is favorable to the formation and maintenance of fog.