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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 ...
In a cold occlusion, the air mass overtaking the warm front is cooler than the cold air mass receding from the warm front and plows under both air masses. In a warm occlusion, the cold air mass overtaking the warm front is warmer than the cold air mass receding from the warm front and rides over the colder air while lifting the warm air. [2]
The inability of an air mass to resist vertical motion. In a stable atmosphere vertical movement of air is generally difficult, whereas in an unstable atmosphere vertical disturbances can be quite exaggerated, resulting in turbulent airflow and convective activity that may lead to extensive vertical clouds, storms, and severe weather.
A ball on the top of a hill is an unstable situation. In dynamical systems instability means that some of the outputs or internal states increase with time, without bounds. [1] Not all systems that are not stable are unstable; systems can also be marginally stable or exhibit limit cycle behavior.
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]
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 ...