Figure 2. Schematics illustrating two idealised stratifications having the same potential energy (PE), but a different partition APE/BPE. Intuitively, the fluid on the left is purely static and has no APE. No motion is expected to develop. The fluid on the right, however, is clearly unstable, as intuitively, one expects the fluid on the top right to start sinking owing to being denser than the other parcels. The fluid on the right has APE, while the fluid on the left has none, despite the two fluids having the same PE. Reproduced from Hughes et al. (2009).