Departure day at RIAT was a bit overcast, much like the majority of the show.  The damp atmosphere did have the positive effect of meaning many of the more powerful prop aircraft were pulling vortices from the tips of their propellers.  This was most obvious earlier in their take off runs but you could get a  pretty good view of it even head on from where I was sitting in the FRIAT stand.  Here is one of the Hercs that was beating the air into submission.