Haar

This is not a word BBC Scotland would use, or likely know, sitting as they do in Glasgow. and rarely coming out. It's derived (*) probably from a Low German or Dutch dialect word akin to Dutch dialect harig damp, misty, Middle Dutch hare sharp wind, piercing cold, Frisian harig misty, Old Norse hārr gray, hoar. The BBC call it sea fog.

All day it has been rolling in and out, latterly much thicker than this, sitting like a great, damp bolster atop and between the Sutors, the two hills lying on the north and south sides of the Cromarty Firth where it joins the sea. Move around and you switch almost instantaneously between bright sunlight and clammy darkness.

On the Cromarty seafront you could barely see 100 metres out to sea, so thick it was. It was really odd sitting in our house, later on in bright sunshine, hearing two ocean liners in turn making their way slowly out into the North Sea, foghorns blaring, yet only at the end of the street.

* Etymology courtesy of Merriam-Webster