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Tuesday, May 17, 2016

Absolute & Conditional Instability

Instability is a race to get cold between the parcel and the environment, and we want to environment to win. We could help the environment win by making the environment cool more slowly and / or make the parcel cool at a slower rate. The parcel method, for example, talks about the parcel being a hypothetical box that does not allow any transfer of heat in or out but, allows only adiabatic temperature changes.

The stability of the parcel is dependent on the parcel’s motion after a forced displacement. As the parcel undergoes adiabatic change, its temperature is compared to the surrounding environment to relate differences in density. If the parcel returns to its original position it is considered stable, whereas if the parcel continues moving away from its original position it is considered unstable. Moreover, if a parcel is displaced but remains at its new position it is considered neutral.

Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (ГE) exceeds the dry adiabatic lapse rate (ГD) [i.e. ГE > ГD]. Whereas, absolute stability occurs when the environmental lapse rate (ГE) is less than the wet adiabatic lapse rate (ГW) [i.e. ГE < ГW]. However when the environmental lapse rate (ГE) falls between the wet adiabatic lapse rate (ГW) and the dry adiabatic lapse rate (ГD) [i.e. ГW < ГE < ГD] the atmosphere is considered conditionally unstable, as you can see from the picture below



On the other hand, especially with regard to the potential for severe storm development, another type of stability becomes important: potential instability. While, static stability (discussed above) considers what happens to a small parcel (box) of air when lifted or lowered while the surrounding air is kept in place, potential instability contemplates what happens when an entire layers of air are displaced upward [i.e. a mass of warm air displaced upward by the movement of a cold front].


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