How Air Parcels Work
- 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.
Why Air Parcels Are Such a Big Deal
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.
- 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.
Key Points
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 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.
- 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.
Adiabatic Processes and Air Parcels
What Abaout Diabatic Procceses?
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.
- 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.
The Bottom Line
- 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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