Abstract: Based on the cases of the southern hemisphere tropical cyclone 7619 and the northern hemisphere tropical cyclone 8111, the variations of low level large scale zonal wind field and the intensity of the crossing-equator wind in the two different original zones of tropical cyclone 7619 and 8111 are calculated. The day-day variations of low level vorticity field and the vertical wind shear fields in troposphere, and the stratified thermo-hygro fields are calculated too. Although the two tropical cyclones are from different hemisphere but their atmosphere physical fields and synoptic situations show some common characteristics in the beginning and ending stages. It is indicated that as the low level large scale streamline convergence in the tropical cyclone original zone is intensified, the horizontal wind shear becomes big quickly which is the main cause for tropical cyclogenesis. A diagnostic formula is provided and the predictability for tropical cyclogenesis and tropical cyclolysis by the way of synoptical quantiative analysis is discussed here.