There is a degree C drop in temp for roughly every 100m of altitude gained, so Snowdon being just over 1000m would be roughly 10degrees lower temp than at sea level.
I haven't ventured outside so I have no idea how warm/cold it is out there so I'll hazard a guess that it will be cold on the top of Snowdon. Maybe even really cold, to the point of needing a flask of hot chocolate and a digestive.
Anyway, I'm heading up Kilvey now, which will have a 2dgree drop in temp, putting it just below the Swansea tropical thermocline....