Based on the theory of dissipation structure,the calculation formula of atmospheric Wet entropy and the conception of relative wet entropy have been presented. The formula of general entropy flow which is easy to be applied in atmosphere has been calculated. Two weather processes of heavy rain and rain gust in Hebei Province were taken, the entropy evolution and its association with the placement of severe convection storm and with the placement of heavy rain of typhoon low have been analysed. The common ground and difference of the two heavy rain events have been presented. The moving law of typhoon low has been recognized.