Abstract: Based on the theory of Ertel potential vorticity, the isentropic potential vorticity maps and vertical potential vorticity profiles for two summer depressions over the Changjiang-Huaihe Valley are analysed. After discussing a possible mechanism for the genesis and development of such systems and their difference from typical extratropical cyclones, a conceptual model for their activities is proposed: A mid-level, weak disturbance originated from around the Qinghai-Tibet Plateau may cause heavy precipitation under favourable conditions, latent heat release in the mid-troposphere leads to downward extension of cyclonic circulation and a wave on the quasi-stationary front. This weak cyclone can develop substantially and become a typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic surfaces into the region of interest.