The air parcel expands as it rises and this expansion, or work, causes the temperature of the air parcel to decrease. Imagine that parcels of _dry air_ go up inside the pipe that goes vertically from 0 to 5000 meters. Suppose that they go up because they are slightly warmer that the air outside. Situation 1 The pipe internal diameter grows from bottom to the top so the area of cross-section grows proportionally to the decline of pressure outside. In this case the speed of parcels is constant along the pipe. The parcels expand. Which mechanical work will they do and will their temperature decrease ? Situation 2 The pipe internal diameter is constant. In this case the speed of parcels grows along the pipe. The parcels expand up? Does this mean that the parcels do mechanical work of pushing upper parcels up? Does this mean that mechanical energy of parcels grows at the expense of their internal energy ? Will the parcels be colder then in Situation 1 ?