Dunno, but you can almost predict a haar whenever there's a south-easterly along the east coast in the summer. The lines of equal sea temperature (isotherms?) run out north easterly from the coast. The August plot shows a line at 17C extending north-easterly from the Thames Estuary, then others at 16C from Lowestoft, 15C from Grimsby, 14C from Whitby and 13C from Berwick. Look in the Admiralty North Sea Pilots. So a south-easterly comes across sea several degrees warmer than the sea at the coast; the air picks up moisture over the warmer sea then drops it as it cools down, giving the mist/fret/haar/roke/what-have-you. It's one of those classic situations that is taught (or ought to be) in the Met section of every shore-based sailing course.