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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
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Oceans, on the other hand, are heated from above, and are usually stably stratified. Only near the poles does the coldest and saltiest water sink. The deep ocean waters slowly warm and freshen through internal mixing (a form of double diffusion), and then rise back to the surface.
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In engineering applications, stable stratification or convection may or may not be desirable. In either case it may be deliberately manipulated. Stratification can strongly affect the mixing of fluids, which is important in many manufacturing processes.
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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
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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,
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Vertical temperature gradient cause by stable stratification of air inside a room. Note hot air rising convectively from the person; bodyheat temporarily disrupts the stable stratification.
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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.
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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
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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
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Stable stratifications can become unstable if layers change density. This can happen due to outside influences (for instance, if water evaporates from a
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tend to preserve stable stratification; the higher layers float on the lower ones. In unstable stratification, on the other hand, buoyancy forces cause
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Sometimes, two physical properties diffuse between layers simultaneously; salt and temperature, for instance. This may form diffusive layers or even
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Maiti, D. K.; Gupta, A. S.; Bhattacharyya, S. (1 December 2008). "Stable/Unstable
Stratification in Thermosolutal Convection in a Square Cavity".
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Xu, Duo; Chen, Jun (December 2016). "On the mixing models for stratified flows subjected to concomitant stable and unstable stratifications".
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Solar energy passes as visible radiation through the air, and is absorbed by the ground, to be re-emitted as heat radiation. The
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are a weather event which happens whenever an area of the lower atmosphere becomes stably-stratified and thus stops moving.
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Typical mixing pattern for many lakes, caused by the fact that water is less dense at the freezing point than at 4 Celsius.
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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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heats that layer from within). Outdoor air is thus usually unstably stratified and convecting, giving us wind.
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Not all mixing is driven by density changes. Other physical forces may also mix stably-stratified layers.
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trace the Kelvin-Helmholtz waves between two thermally-stratified layers of the atmosphere.
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relies on selectively encouraging and disrupting stable stratification to cool rooms.
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A simple model of an unstable stratification converting to a stable one (in
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may form at the interface. These patterns are also seen on other planets.
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When two stably-stratified layers are moving relative to one another,
452:"NASA's Solar Dynamics Observatory Catches "Surfer" Waves on the Sun"
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deliberately creates unstable stratification of the air in a room.
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Stratification is commonly seen in the planetary sciences.
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plumes (with "mushroom" heads) in both colours/directions.
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fluids, like oil and water, or the wax and water of a
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is therefore heated from below (UV absorption in the
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471:"Stably Stratified Atmospheric Boundary Layers"
450:Zell, Holly; Fox, Karen C. (30 December 2014).
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434:References
352:Examples:
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