Parcel Displacements
A key assumption of the parcel theory is that displacements are small, confined to the 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.
For example a parcel in a weak, low-level inversion is stable. However, a thick unstable layer may cap that low-level inversion. If this is the case, a parcel lifted to a point above the inversion will become unstable and will accelerate upward.