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It is a form of fluid instability found in thermally stratified atmospheres in which a colder fluid overlies a warmer one. When an air mass is unstable, the element of the air mass that is displaced upwards is accelerated by the pressure differential between the displaced air and the ambient air at the (higher) altitude to which it was displaced.
This makes moist air generally less stable than dry air (see convective available potential energy [CAPE]). The dry adiabatic lapse rate (for unsaturated air) is 3 °C (5.4 °F) per 1,000 vertical feet (300 m). The moist adiabatic lapse rate varies from 1.1 to 2.8 °C (2.0 to 5.0 °F) per 1,000 vertical feet (300 m).
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 model also assumes a constant static stability parameter and that fluctuations in the density of the air are small (obeys the Boussinesq approximation). Structurally, the model is bounded by two flat layers or “rigid lids”: one layer representing the Earth's surface and the other the tropopause at fixed height H. To simplify numerical ...
Unstable areas are in yellow (slightly) and red (highly) while the stable zone is in blue. The lifted index (LI) is the temperature difference between the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa . The ...
Classes range from 1 (most unstable) to 7 (most stable). The Turner stability class system was devised by D. B. Turner as a modification of the Pasquill stability class system. [1] The following table is used to determine the Turner stability class for a given wind speed and net solar radiation:
Weather radar loop showing intense snow bands (lighter color) due to CSI ahead of a warm front.. Conditional symmetric instability, or CSI, is a form of convective instability in a fluid subject to temperature differences in a uniform rotation frame of reference while it is thermally stable in the vertical and dynamically in the horizontal (inertial stability).
This means that unstable air is now stable when it reaches the equilibrium level and convection stops. This level is often near the tropopause and can be indicated as near where the anvil of a thunderstorm because it is where the thunderstorm updraft is finally cut off, except in the case of overshooting tops where it continues rising to the ...