Sea smoke, arctic sea smoke, frost smoke, or steam fog is present when the surface of the sea has a steaming or smoky appearance. This fog is often patchy and extends to a limited height above the surface, with good visibility at bridge level but poor from the upper deck.
The condition is caused by the movement of cold air over a warmer surface, the temperature difference usually being of the order of 10°C, although given favourable wind conditions it may occur with smaller differences. The air immediately above the surface is heated and becomes saturated through evaporation from the surface. It ascends and mixes with colder unsaturated air above. Since the mixture is supersaturated, condensation occurs and the water droplets form sea smoke. As the air is heated by the underlying surface, the environmental lapse rate will be that of an unstable atmosphere.
The wind speed associated with the formation of sea smoke may vary from very low to gale force. Higher speeds are more favourable when the temperature difference is small, as they ensure a continuous supply of cold air immediately above the surface.Off the east coasts of the North American and Asian continents sea smoke occurs during the winter months, when cold air from the continent passes over estuaries, coastal waters, e.g. St. Lawrence
Seaway and adjacent ocean areas. During winter it occurs in the Baltic Sea which is surrounded by a colder land mass, and in higher latitudes it is associated with cold winds from the Arctic Basin and the ice covered sea areas to the south. In lower latitudes it occurs occasionally in the Gulf of Mexico and off Hong Kong.
As the air temperature is low, the droplets forming sea smoke are often supercooled, and icing occurs when the droplets freeze on contact with those parts of a vessel which are below 0°C.