page_title: Stable and unstable stratification
section_title: Destablization and mixing
hierarchical_section_title: Stable and unstable stratification / Destablization and mixing
caption_attribution_description: English: created by author
context_page_description: Stable stratification of fluids occurs when each layer is less dense than the one below it. Unstable stratification is when each layer is more dense 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 more-dense layers above, and the more-dense 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.
context_section_description: Stable stratifications can become unstable if layers change density. This can happen due to outside influences (for instance, if water evaporates from a freshwater lens, making it saltier and denser, or if a pot or layered beverage is heated from below, making the bottom layer less dense). However, it can also happen due to internal diffusion of heat (the warmer layer slowly heats the adjacent cooler one) or other physical properties. This often causes mixing at the interface, creating new diffusive layers (see photo of coffee and milk).
  Sometimes, two physical properties diffuse between layers simultaneously; salt and temperature, for instance. This may form diffusive layers or even salt fingering, when the surfaces of the diffusive layers become so wavy that there are "fingers" of layers reaching up and down.
  Not all mixing is driven by density changes. Other physical forces may also mix stably-stratified layers. Sea spray and whitecaps (foaming whitewater on waves) are examples of water mixed into air, and air into water, respectively. In a fierce storm the air/water boundary may grow indistinct. Some of these wind waves are Kelvin-Helmholtz waves. 
  Depending on the size of the velocity difference and the size of the density contrast between the layers, Kelvin-Helmholtz waves can look different. For instance, between two layers of air or two layers of water, the density difference is much smaller and the layers are miscible; see black-and-white model video.
caption: Typical mixing pattern for many lakes, caused by the fact that water is less dense at the freezing point than at 4 Celsius. Lake stratification is stable in summer and winter, becoming unstable in spring and fall when the surface waters cross the 4 Celsius mark.
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