Aerocat/Tinstall
This analogy of the moving walkway is perfect (for me anyway). I could understand the comments about the aircraft moving with respect to the airmass, however made a logical jump to thinking that in that case there can be no problem associated with moving from one air mass to another because you continue to move with respect to the new airmass - and that is something I knew to be rubbish. The walkway analogy makes it very clear.
Making a fool of myself in public is not something I like doing, but this time, I am actually glad I did because I now understand the matter much more. Thanks for the correction guys.