Everything about Air Parcel totally explained
===Definition===An
air parcel is an imaginary volume of air used by
meteorologists to conceptualize the
thermodynamic fluid motions of the
atmosphere for use in
weather forecasting. For mathematical simplicity, an air parcel is usually considered a rigid cube which has limited interactions with surrounding environmental air. The dimensions of this parcel are determined by the atmospheric scale under study.
The importance of the concept of air parcels in
meteorology lies in its ability to assist meteorologists in conceptualizing how areas of an atmosphere will feature rising and sinking motions, the magnitude of those motions, and the possibility of ensuing
clouds and
precipitation. Many older
numerical weather prediction models used the conceptual models of air parcels.
Buoyancy
The primary concern of air parcels for meteorologists is whether that'll rise, sink or remain steady. To understand this motion, it's helpful to consider thermodynamic
buoyancy equations. One such equation can be derived as follows.
Let the weight of the parcel be
To summarize in simpler terms, there are four major factors which affect the buoyancy of air parcels. Pressure perturbation decelerates parcels because moving parcels have to "push" surrounding air out of the way. Density differences between the air parcel and its environment can accelerate or decelerate the parcel. Precipitation within the parcel can act a drag on upward motions. Lastly, mixing of the parcel air with environmental air acts to decrease buoyancy.
Applications
When using a
Skew-T chart (a
Tephigram chart can be used for this as well) to diagnose the atmosphere, meteorologists assume that air interacting with the environment will behave as an air parcel (that individual parcels of air won't significantly interact with the environment). A meteorologist can then trace a parcel up or down the appropriate
adiabat to determine the desired variable, which can be
temperature,
pressure,
moisture content, and so forth.
The notion of air parcels is partially confirmed by examining convective plumes within
cumulus congestus clouds. Individual columns of rising air can be observed in such clouds. In time lapse photography, areas of convection can be observed, and the individual terms of the buoyancy equation can be observed in action (for example warmer air rising, entrainment of environmental air causing decay, etc). The figure on the right demonstrates how a rising air parcel can create a localized cloud in the beginning stages of a thunderstorm.
Further Information
Get more info on 'Air Parcel'.
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