Air parcel
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An air parcel is a meteorologically defined chunk of air in the atmosphere. How large or small this chunk is depends on scale you are looking at. Meteorologists study the movements and changes in air parcels over time as one tool in weather forecasting.
Conceptually, one could imagine that the air inside a hot air balloon is an air parcel. An Air Parcel has certain properties; heat and moisture being two important ones. With no outside influence, no change to the parcel. When heat is added to a parcel, the parcel becomes warmer than the surrounding environment/atmosphere/air, the parcel will rise. Likewise, when it becomes cooler than the surrounding environment, it will sink. In parcel theory, no mixing will occur between the contents of the parcel and it's surroundings. All changes within the parcel happen due to expansion and contracting as pressure and/or temperature changes outside of the parcel. rising always results in expansional cooling within the parcel while sinking results in compressional warming.
A meteorologist can look at a Skew-T chart (a Tephigram chart can be used for this as well) and determine how a parcel of air will act, which in turn can be used to forecast the weather.
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