Stable and unstable stratification

A simple model of an unstable stratification converting to a stable one (in immiscible fluids, like oil and water, or the wax and water of a lava lamp). Note Rayleigh–Taylor instability plumes (with "mushroom" heads) in both colours/directions.

A human hand heating air. The heated air is underneath unheated air, an unstable stratification, so the hand-heated air rises and the cool air sinks, causing convection.

A stably-stratified beverage of cold milk, warm coffee, and cream. The least dense layer is on top. The milk and coffee are slowly mixing to form new diffusive layers, visible in intermediate shades of brown, as the milk warms and the coffee cools at the interface.

Diffusive layers may internally be homogeneously-mixed, but with each layer different from the next. This leads to stair-step profiles in physical properties (here, temperature and salinity; in the previous photo, colour).

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Stable stratification of fluids occurs when each layer is less dense than the one below it. Unstable stratification is when each layer is denser than the one below it.

Buoyancy forces tend to preserve stable stratification; the higher layers float on the lower ones. In unstable stratification, on the other hand, buoyancy forces cause convection. The less-dense layers rise though the denser layers above, and the denser layers sink though the less-dense layers below. Stratifications can become more or less stable if layers change density. The processes involved are important in many science and engineering fields.