Abstract: This paper deals with the formation and classification of the fog in Shanghai urban area. By analysing the records 27 fog days during 1984, it is found that there are four types of fogs: frontal fog (accounting for 40.7%), radiation fog (accounting for 29.7%), advection radiation fog (accounting for 18.5%), and advection fog (accounting for 11.1%). Each type is explained in detail with some examples. In addition, using the Shanghai fog day data during 1951—1988, it is pointed out that : (1) In Shanghai urban area, the mean fog day becomes fewer, (2) Shanghai urban fog is closely related to the urban moisture island, (3) The urban “pollutant fog” with the low relative humidity frequently occurs in Shanghai.