An analysis of the heavy fog which appeared in southern Hebei Province from the mid-night of 28 to the noon of 29 November in 2003 is made.The results show that the physical elements relating to stability are the indexes not only of severe convection process,but also of heavy fog.Fog is a weather phenomenon which comes forth in steady atmosphere. From the stratification and change of the atmosphere, a heavy fog appearing or not,as well as the growing or vanishing time can be decided.In addition,the humidity's accumulation is a cause affecting the heavy fog making also.