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What is an Air Parcel?

How Air Parcels Work

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    Why Air Parcels Are Such a Big Deal


  • The term 'air parcel' is just everywhere in meteorology. If you are reading a meteorology textbook, you will come across it many times.
  • This is because the concept of a parcel allows you to understand what happens when the atmosphere or part of it becomes unstable.
  • By understanding parcel theory, it will be easier for you visualize what's happening when unstable air is rising.

What is an Air Parcel Exactly?

  • An air parcel is like a pocket of air. The conditions are the same everywhere inside of it.
  • What is more, it is usually assumed that outside air does not mix in with the air inside the parcel — like a sealed box, so to speak.

    Key Points

  • 1. An air parcel is an imaginary pocket of air that helps meteorologists understand how air that is more unstable or stable behaves differently compared to the rest of the atmosphere.
  • 2. You can also think of it as an imaginary pocket, bubble, box or volume of air.
Parcel A is the true air parcel because the pressure is the same everywhere inside of it.
  • An air parcel doesn't have a specific size. But, it is large enough to have a significant number of molecules.
  • This is essential because you can't have an imaginable atmosphere with just one or two molecules. Why, a mere liter of air has myriads of particles!
  • But, it is small enough so that all properties such as pressure, temperature and humidity are the same anywhere inside it.

    Adiabatic Processes and Air Parcels


  • Adiabatic processes assume that there is no mixing between air outside and inside the parcel.
  • Therefore, for adiabatic processes, it is important to think of an air parcel as a sealed box that does not mix with the air outside of it.
  • But, for all cases, adiabatic or not, the air must be big enough to contain a large number of molecules to have an imaginable atmosphere.
  •  
  • What Abaout Diabatic Procceses?


  • Unlike an adiabatic process, a diabatic process assumes that air inside the parcel does mix in with the air outside.
  • An example of that would be pocket of cold air moving over a warm body of water. Of course, through conduction or contact, the air inside the parcel would get warmer.
  • Even in such cases, though, the parcel must be big enough to contain many molecules, otherwise it would not be an imaginable situation.

A Simple Way to Imagine an Air Parcel

  • As already mentioned, parcel theory allows you to understand what's happening when an unstable pocket of air is rising.
  • There are different examples you can use to understand air parcels. One example are thermals or updrafts.
  • An updraft or thermal is fitting as an air parcel, since it is a pocket of air that is rising.
  • The warm air parcel or thermal will continue to rise as long it remains less dense than its surroundings.

    The Bottom Line

  • As long as a pocket of air is is more unstable or stable compared to its surroundings, you can consider it as an air parcel.
  • In a majority of cases, parcel theory is used to show the behavior of unstable air compared to its surroundings.

    Get Creative with Air Parcels !


  • It would be an injustice to only use thermals to visualize air parcels.
  • Here are some different ways you can visualize air parcels:

  • 1. Air bubble (parcel) rising from a pool of water.
  • 2. A parcel of warm air rising alongside a cold front.
  • 3. A parcel of air pushed upward on a mountain (orographic lifting).
  • 4. A parcel of moist air pushed upward alongside a dry line (moist air is less dense than dry air).

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