PeteG,
Being a bit of a sceptic, I'd say that was a local temperature inversion, where the temperature in the dip was lower than the temperature on the rise in the field, i.e. a "frost" hollow formed, where the significantly cooler air below mixed with the warmer air above to form a fog on the boundary, and the air movement showed it up as a swirling motion.
This is fairly regularly seen on mountains in the autumn/winter months, where, unbelieveably, the temperature at 3,000 feet can be about 6 or 8 degrees above that in the valley. See the link below for the most recent:-
http://www.leaney.org/lake_district_walk.php?walk_id=641
Regards,
TE.