In weather forecasting, there are some improper uses of the concept of stratospheric dry air intrusion into troposphere deduced from the isentropic potential vorticity (PV) analyses. This paper reviewed the history of weather forecast from pressure change to vorticity change, and discussed the origin of PV concept. Furthermore, based on the definition, mathematical expression, physical meaning, and computation of PV, this paper showed that PV value is mainly determined by the vertical gradient of potential temperature, and PV distribution reflects tropopause distribution, and thus indirectly reflects the distribution of polar air mass, front, upper-level trough and jet stream. The trajectories and model simulation both show that the PV anomaly in the lower atmosphere is resulting from the strong deepening of surface cyclone and the feedback of latent heat, instead from dry intrusion. This paper also pointed out that the law of PV conservation cannot be used to replace the dynamic mechanism of baroclinic disturbance development. The misuse of PV is from the heuristics of PV conservation and the confusion between trajectories and streamlines used in fluid mechanics.