Based on the infrared images and conventional data,a Squall weather process on 18,June 2001 is diagnosed.The results show that since a longitudinal Inner Mongolia low vortex turned into latitudinal pattern in the eastward and southward process,a severe northerly cold flow went down to the northern Henan Province,and then,the air was getting more instable.In the lower layer,because the inflow of a tongue of potential pseudo-equivalent temperature made air warmer and wetter,a potential instablility stratification was formed.The positive vorticity advection in the southwest of the low vortex in the upper layer and wind shear between Zhengzhou,Henan Province and Xingtai,Hebei Province provided the dynamical conditions for the occurrence of severe convective weather.