The haar of north‐east Scotland

Case studies of three sea fog (haar) situations off the coast of NE Scotland are presented. The studies are based on aircraft and mini‐sonde observations and satellite imagery, and are used to assess the roles of physical and dynamical factors in developing and advecting sea fog in order to improve guidance to forecasters at coastal outstations. Two fogs had formed 2–3 days previously west of Ireland and had been advected round the N coast of Scotland, but the second fog had lifted into stratus by the time the aircraft had reached it. The third fog was warmer than the sea and appeared to be in the development/advection stage characterized by much stronger winds and lower liquid water contents. Analysis and some modelling of the one‐dimensional heat and water budgets of sea fog suggest that after a period of development during transport over a relatively cold sea, initially ‘warm’ advection fog tends to become a diurnally modulated, self‐maintaining ‘cold’ radiation fog held at a temperature 1–2 degC below sea surface temperature. In other words, radiative cooling has depressed the temperature of the fog far enough below sea surface temperature for this cooling to become balanced by latent and sensible convective heat input from the sea surface. The aircraft observations demonstrated the convective control of the vertical structure of ‘cold’ sea fog, and the influence of diurnally forced wind systems (in this case, a heat low formed over the Scottish mainland) in moving and moulding the fog.