Steam Fog

Steam Fog

Arctic Sea Smoke

Evaporation Fog

Sea Smoke

Steaming Fog

Frost Smoke



Steam Fog, also known as Steaming Fog, Evaporation Fog, Frost Smoke, Sea Smoke or Arctic Sea Smoke, occurs when evaporation takes place into cold air lying over warmer water. It is usually quite shallow. Steam Fog should not be confused with Advection Fog which is formed in warm moist air over a cold surface.

Steam Fog is named by analogy with the condensed vapour, or steam, which appears above water which is heated. Invisible vapour is given off from the water but is almost immediately re-condensed as it comes into contact with the colder air. The air has to be much colder than the water, typically 0 °C or below, so that a comparatively small amount of moisture can produce supersaturation, otherwise the heating process (of condensation) will outweigh the tendency towards saturation. For the Steam Fog to persist, a marked surface temperature inversion is required in the air before it moves over the sea or inland water bodies so as to preclude the lapse rate becoming unstable through a deep layer allowing mixing and evaporation.

This type of fog is usually only formed over water surfaces near to a source of cold air such as frozen ground or ice sheets in polar regions. One classic occurrence is following the sudden break up of sea ice to expose relatively warm water. Although steam fog is usually very shallow, in the steep sided fjords along parts of the Icelandic and Norwegian Coasts and similar environments elsewhere, steam fog may reach a depth of 500 feet or more and drift over adjacent land areas. This can cause coastal airports in northern latitudes to quickly change from unlimited visibility to dense fog as the Steam Fog drifts across the from the water.

Sea fog over Lake Superior

Sea Fog over Lake Superior (Duluth, Michigan), January 2016 [Source: Wikicommons. Author: 2ndDaughter75]

Whilst relatively rare in temperate latitudes, cold air which collects in and then drifts down large river valleys and out over a relatively warm sea surface in very light wind conditions can occasionally lead to the formation of smaller and much shallower areas of this type of fog in winter.

In a more ephemeral context but by exactly the same process, many people will have seen the steaming of tarred road surfaces or bitumen roofs in sunshine after rain.

Evaporation Fog formed by cool air flowing over a lake; NOAA's National Weather Service (NWS) Collection [source: wikicommons]

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