Smith - good point on the haar - I recently spent 3 years in eastern Scotland and the Forth / Tay estuaries could get engulfed in a haar for days at a time and these had a tendency to form with little notice. The haar looks like quite a light layer when you're underneath it, but when you go up through a hole and get on top, it is a pretty uniform stratus which is usually no more than 500 - 1000 ft thick, but conceals the ground very effectively!