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 fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

. 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, 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 are aerosol particles formed from the ocean, mostly by ejection into Earth's atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA ha ...

and whitecaps (foaming

whitewater Whitewater forms in a rapid context, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and ...

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, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...

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 Lower atmosphere is a collective term sometimes used to refer to various layers of the atmosphere of the Earth and corresponding regions of the atmospheres of other planets, and includes: * The troposphere The troposphere is the first and lowest ...

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 absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in rela ...

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

Temperature inversion In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. No ...

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 is the separation of water in layers. Two main types of stratification of water are uniform and layered stratification. Layered stratification occurs in all ocean basins. Stratified layers act as a barrier to the mixing of water, 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 generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of th ...

Atmospheric stratification The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

, 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 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 from year to year, but t ...

, caused by the unstable stratification of the atmosphere *

Thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temper ...

, circulation in the oceans despite stable stratification. * Stratified flows (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 central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and ...

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 from ...

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

References

{{Reflist Fluid dynamics