Fog
Though there are at least five types of fogs—upslope fog, frontal fog, radiation fog, advection fog, and convection fog—only the latter two types are common at sea.
Advection fog forms when warm, moist air moves over a cold surface, but rarely supercools. Rime ice formation is relatively rare in advection fogs, and when it does occur, extreme icing rates are unlikely.
Convection fog forms when water temperatures are higher than the air passing over them. It is also called evaporation fog, or sea smoke, as air heated by the relatively warm water surface rises from the water surface as “parcels” of air. The warm, humid rising air parcels cool as they rise and reach the dew point, forming fog. If the air temperature is well below 0°C, the fog supercools and will freeze when striking shipboard objects.
Makkonen [1] reviews studies explaining the formation of convective fog at sea. Convection fog can form to thicknesses of up to 100 m above the sea surface, depending upon the sea surface temperature, the air temperature, and the air relative humidity. If sea water temperatures are between –2 and 15°C, air temperature at a relative humidity of 95% must be 6°C lower than the sea temperature for convection fog to form, and 16°C lower than the sea surface when relative humidity is 0%. There is also a linear trend between the depth of convection fog and the difference between the sea surface and the air temperature [1]. Generally, as the air becomes colder than the sea surface temperature, from about –2 to –15°C, fog depth increases respectively from about 1 m thick to about 25 m or more thick.
References
- ↑ 1.0 1.1 Makkonen, L. 1984. Atmospheric icing on sea structures. Monograph 84-2. Hanover, NH: US Army Cold Regions Research and Engineering Laboratory.
- ↑ Charles C. Ryerson. April 2013. Icing Management for Coast Guard Assets. Cold Regions Research and Engineering Laboratory. ERDC/ C R R E L TR-13-7.