Adiabatic Expansion Question

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From TPR

The volume of air increases as it rises through the atmosphere. Atmospheric scientists will often assume that the air behaves as an ideal gas and that there is no transfer of heat through the boundaries that describe the system.

Which one of the following best characterizes the process of air rising through the atmosphere?

The answer is:

Adiabatic, exergonic expansion


Their reasoning is:
The passage states that scientists assume the process is adiabatic (eliminating choices C and D). Since air rises spontaneously and does work on the surroundings, the process is exergonic. The correct answer is B.

I don't agree with the bold portion! This situation is analogous to a gas expanding to occupy a vacuum. No external pressure is applied, so there is no work done.

In this case as the external pressure gradually decreases, the volume gradually increases to occupy the maximum volume. No work is done here either.

Is my logic correct? Or am I missing something here?

Thanks!

 

[milski]

The (parcel of) air does not expand in vacuum as it rises, it expands against the atmosphere. The fact that the outside pressure changes does not mean that work is not done, if the volume changes, work is being done.