Two observations that complement the conversation here.
From time to time at work, I get to see air being evacuated from vacuum chambers and it is always amazing how much water vapor condenses out of the air during the pressure drop. A cloud bank forms in the chamber. My understanding is that the temperature drops from the pressure drop.
Another demonstration of the pressure drop causing condensation can be seen sometimes on the tip of an airplane wing where no combustion is going on. The pressure increase as the wing approaches changes to a pressure drop on the other side and you can see the vapor trail.
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