In reply to The Lemming:
Wind direction is not constant with altitude, so even without local disturbances, the clouds often don't move in the same direction as the wind at ground level. To those in the know, it gives information as to how the weather will change. Veering and backing are terms related to this, which I no longer remember what they mean.
Basic meteorology is good fun and really adds another level of detail to a day out.
Shame I can't find my weather book.
Post edited at 21:22