One might mention the obvious fact that just because the dreaded Ash Cloud is not over one's departure airport, it doesn't mean your aircraft will either arrive or depart.......as was the case with the i/c 738 to depart from GRO as FR9008 today. Came in an hour late, was advised to stay below FL19 on departure, but in the end FR9008 never departed at all.
Ryanair's own website showed the flight as cancelled, but a forlorn passenger in my car on the way home accessed 'arrivals' on Gatwick's computer which showed FR9008 landing on time in the UK. Most wry.