The Rayleigh–Taylor instability is another application of hydrodynamic stability and also occurs between two fluids but this time the densities of the fluids are different. Due to the difference in densities, the two fluids will try to reduce their combined potential energy. The less dense fluid will do this by trying to force its way upwards, and the more dense fluid will try to force its way downwards. Therefore, there are two possibilities: if the lighter fluid is on top the interface is said to be stable, but if the heavier fluid is on top, then the equilibrium of the system is unstable to any disturbances of the interface. If this is the case then both fluids will begin to mix. Once a small amount of heavier fluid is displaced downwards with an equal volume of lighter fluid upwards, the potential energy is now lower than the initial state, therefore the disturbance will grow and lead to the turbulent flow associated with Rayleigh–Taylor instabilities.
This phenomenon can be seen in interstellar gas, such as the Crab Nebula. It is pushed out of the Galactic plane by magnetic fields and cosmic rays and then becomes Rayleigh–Taylor unstable if it is pushed past its normal scale height. This instability also explains the mushroom cloud which forms in processes such as volcanic eruptions and atomic bombs.
Rayleigh–Taylor instability has a big effect on the Earth's climate. Winds that come from the coast of Greenland and Iceland cause evaporation of the ocean surface over which they pass, increasing the salinity of the ocean water near the surface, and making the water near the surface denser. This then generates plumes which drive the ocean currents. This process acts as a heat pump, transporting warm equatorial water North. Without the ocean overturning, Northern Europe would likely face drastic drops in temperature.