It might be a bit murky down below, but the air
seems quite dry higher up:

Temps and dew points at 1500Z from
http://www.met.rdg.ac.uk/~brugge/15.html

4.0 -21.1 Aonach Moor
4.1 -18.1 Cairngorm
3.7 -15.0 Cairnwell
2.6 -17.5 Great Dun Fell

Or is this a feature of modern instrumentation -
or a synop-creation bug?

Roger