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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.
Monsoon air masses are moist and unstable. Superior air masses are dry, and rarely reach the ground. They normally reside over maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below, forming what is known as a trade wind inversion over the maritime tropical air mass.
Instability results from difference between the adiabatic lapse rate of an air mass and the ambient lapse rate in the atmosphere. [2] If the adiabatic lapse rate is lower than the ambient lapse rate, an air mass displaced upward cools less rapidly than the air in which it is moving. Hence, such an air mass becomes warmer relative to the ...
The Bergeron classification is the most widely accepted form of air mass classification. Air mass classifications are indicated by three letters: [3] [4] Fronts separate air masses of different types or origins, and are located along troughs of lower pressure. [5] The first letter describes its moisture properties, with c used for c ontinental ...
A third source of lift is wind circulation forcing air over a physical barrier such as a mountain (orographic lift). [10] If the air is generally stable, nothing more than lenticular cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers or thunderstorms may appear. [18]
The warm air mass behind a warm front is not only warmer, but often (but not always) also higher in humidity than the colder air preceding it. Because of a warm air mass’s higher temperature and thus lesser density, mixing between the two air masses is unlikely. Being light, the warm air mass is unable to displace the cooler air mass and ...
Under certain stable hydrostatic and inertial conditions, slantwise displacement may, however, be unstable when the particle changes air mass or wind regime. The figure on the right shows such a situation.
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 ...