Determining Stability » The Parcel Method Parcel Displacements
A key assumption of the parcel method is that displacements are small, confined to levels adjacent to the parcel level. However, to accurately determine the stability of the real troposphere, the entire profile from ground to tropopause needs to be examined.
The animation shown here provides an example. There is a weak, low-level inversion, and a parcel in that low-level layer is stable. However, a deep unstable layer caps the low-level inversion. Thus, in this case, a parcel lifted to a point above the inversion will become unstable and will accelerate upward.