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 Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

. 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.

Destablization and mixing

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 Salt fingering is a mixing process, example of double diffusive instability, that occurs when relatively warm, salty water overlies relatively colder, fresher water. It is driven by the fact that heated water diffuses more readily than salty water ...

, 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 Sea spray consists of aerosol particles formed from the ocean, primarily by ejection into Earth's atmosphere through bursting bubbles at the air-sea interface Sea spray contains both organic matter and inorganic salts that form sea salt aeroso ...

and whitecaps (foaming

whitewater Whitewater forms in the context of rapids, in particular, when a river's Stream gradient, gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that foam, froths, making t ...

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 wave In fluid dynamics, a wind wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result of the wind blowing over the water's surface. The contact distance in the direction of the wind is ...

s 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.

Applications

Planetary science

Stratification is commonly seen in the planetary sciences. Solar energy passes as visible radiation through the air, and is absorbed by the ground, to be re-emitted as heat radiation. The lower atmosphere is therefore heated from below (UV absorption in the

ozone layer The ozone layer or ozone shield is a region of Earth's stratosphere that absorption (electromagnetic radiation), absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the a ...

heats that layer from within). Outdoor air is thus usually unstably stratified and convecting, giving us wind.

Temperature inversion In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...

s are a weather event which happens whenever an area of the lower atmosphere becomes stably-stratified and thus stops moving. 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. Examples: *

Stratification (water) Stratification in water is the formation in a body of water of relatively distinct and stable layers by density. It occurs in all water bodies where there is stable density variation with depth. Stratification is a barrier to the vertical mixing ...

Ocean stratification Ocean stratification is the natural separation of an ocean's water into horizontal layers by Density of water, density. This is generally stable stratification, because warm water floats on top of cold water, and heating is mostly from the sun, whi ...

, the formation of water layers based on temperature and salinity in oceans **

Lake stratification Lake stratification is the tendency of lakes to form separate and distinct thermal layers during warm weather. Typically stratified lakes show three distinct layers: the epilimnion, comprising the top warm layer; the thermocline (or metalimnion) ...

, the formation of water layers based on temperature, with mixing in the spring and fall in seasonal climates. *

Atmospheric instability Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric instability encourages vertical motion, which is directly cor ...

Atmospheric stratification The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

, the dividing of the upper reaches of the Earth's atmosphere into stably-stratified layers *

Atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies fro ...

, caused by the unstable stratification of the atmosphere *

Thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. The name ''thermohaline'' is derived from ''wikt:thermo-, thermo-'', r ...

, circulation in the oceans despite stable stratification. *

Stratified flows The flow in many fluids varies with density and depends upon gravity. The fluid with lower density is always above the fluid with higher density (stable stratification). Stratified flows are very common such as the Earth's ocean and its atmosphere. ...

(such as the flow through the Straits of Gibraltar)

Engineering

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. *

Underfloor heating Underfloor heating and cooling is a form of Heating, ventilation, and air conditioning, central heating and cooling that achieves indoor climate control for thermal comfort using hydronics, hydronic or electrical heating elements embedded in a fl ...

deliberately creates unstable stratification of the air in a room. *

Passive cooling Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat fro ...

relies on selectively encouraging and disrupting stable stratification to cool rooms.

References

{{Reflist Fluid dynamics Vertical distributions