Air parcel

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Aero package is a term from the theoretical meteorology and referred to a certain air quantity or an air volume that is large enough to over state variables such as temperature , pressure , density and humidity are described to be able to, but again is small enough to fit in to be considered infinitesimal in mathematical treatment . It is therefore a theoretical concept that is intended to be able to correctly describe small-scale effects in a narrowly limited space, although there is no such limitation.

The term is mainly used when describing adiabatic vertical or advective processes of atmospheric dynamics. It is particularly suitable from a didactic point of view, as it is easier to imagine a certain amount of air and its rise in the atmosphere, for example, than to grasp the merging dynamics of entire air masses .

Parcel method

Transferring the equilibrium conditions of the mechanism to the atmosphere , so one takes place of a body of an air parcel ( Parcel method , engl. Parcel "package").

Before a vertical displacement, the air package has the same density or temperature as the surrounding air. No heat is added to or withdrawn from it, that is, it moves adiabatically , and then no mixture with the surrounding air occurs ( laminar flow ).

Hydrostatic equilibrium on graph paper
Stable atmosphere stratification
The stable atmospheric stratification describes the state of the earth's atmosphere in which the vertical temperature decrease is smaller than the dry adiabatic temperature decrease (TA), i.e. less than 0.98 ° C at 100 meters altitude difference.
Unstable atmosphere stratification
With an unstable atmosphere stratification, the air parcel rises with a temperature decrease of more than 0.98 ° C per 100 m. Because the air parcel is always warmer than the environment, it continues to rise faster and further away from equilibrium.
Neutral atmosphere stratification
With a neutral stratification of the atmosphere (indifferent equilibrium) the air parcel rises dry abiabatically at 0.98 ° C per 100 m. Because of the neutral stratification of the atmosphere, it stays in its new place and does not “fall” back to the starting point.

The equilibria behave analogously for the wet adiabatic ascent, in which the air parcel rises wet adiabatically, i.e. at about 0.5 ° C per 100 m. A distinction is also made here between moisture-stable and moisture-unstable stratification.

In the special case of the conditionally unstable atmosphere stratification, the atmosphere is stable with dry adiabatic ascent, but unstable with wet adiabatic ascent.

Individual evidence

  1. Guide for training in DWD No. 1, German Weather Service .