Getting shots of vapor is a popular thing when shooting fast jets.  An airliner on a damp day can also result in some clouds being formed.  While shooting something special at Oakland, I was getting a few shots to make sure the light metering was working as planned.  A Southwest 737 came in and, as it flared for landing, it had a nice cloud of moisture form over the wing.  Normally these things pop up further out on the approach and don’t make for a good shot but this one was close at hand.