The investigation of mechanisms of nucleation of ice crystals by aerodynamic cooling produced by supersonic airflow is carried out. Three processes are considered to be the principal causes for aerodynamic cooling and nucleation of ice crystals. They are: adiabatic cooling in supersonic airflow, cooling at the cores of vortices around the edge of airflow and entrapment of ambient stationary air into supersonic airflow. It is thermodynamically confirmed that the temperature lowering in supersonic flow depends on the Mach number M there and stagnant pressure Po at a certain stagnant temperature To. The temperature will decrease by more than 6oC as M increases by 0.1. The influence of Po on cooling is shown through the variation of mass flow rates, which increase with Po.Experiments in laboratory have shown that ice-forming rate produced by supersonic airflow increases from 1011 to 1012 /g as M increases from 1.10 to 1.84 at Po= 5 and 6 atm, and increases from 4.3 × 1011 to 10.3 × 1012 /g as the mass flow rate increases from 3.5 to 5.7 g / s and increases from 1.5 to 5.0 atm at M = 1.80 and To= 25oC. In field experiments the ice concentrations of 50 to 200 per liter in about 2000 m3 were measured when air of about 0.5 g were spurted at a Mach number of M = 1.8 into supercooled fog with temperatures between -0.5oC and -4.6oC. These results are compatible with the prediction of aerodynamics.The snapshot taken in experiments represents the detailed structures of vortex motion around a supersonic airflow.