Abstract: The ice-forming number ρi Per gram of the supersonic air flow,was investigated in a cold chamber with fog and temperature of 0——12℃.It was found that ρi had a factor of 1012-1012g-1 as the Mach number increased from 1.1 to 1.84 and the total Pressure was less than 6 atm.And furthemore,ρi increased as the Mach nunber and the total Pressure increased,and the ambient temperature decreased.These results are consistance with the law shown by the supersonic expansion theory.The analyses of the mechanisms of ice crystals generation by the supersonic air flow was made.It was considered that the homogeneous condensation of water vapour and the freezing immediately (condensation-frcezing) is a main Process responsisle for the generation of the ice crystals.Autllors indicate that the application of the supersonic air flow for the weather modification has many advantages (for example,saving expenses,no Pollution,simple operation on the long distances for air Plane).By experimental data,it is Possible to develop a new technic for weather modification.