Steam devil

Description

A steam devil is a small, weakly rotating column of condensed water vapour over water (or rarely over wet land) that has drawn fog into the vortex, thus rendering it visible. Typically, it is associated with steam fog (Arctic sea smoke), a type of fog that develops in very cold air above a relatively warm body of water or very wet land.

Smaller ones can form over ponds, hot springs, and geysers when the air is very cold. Larger ones can form over large lakes and oceans during cold air outbreaks while the water is still relatively warm, and can be an important mechanism in vertically transporting moisture.

Steam devils have only been reported and studied since the 1970s. They are weaker than waterspouts and distinct from them. They are more akin to dust devils in that they develop near the surface and have no “parent” cloud.

Appearance

Smaller steam devils can form over small lakes, especially the warm water in the hot springs of geyser basins. In these cases typical dimensions are a metre or so diameter, but can vary from less than 0.1 to 2 metres, and a height of 2 to 30 metres with a somewhat faster rotation of 60 rpm or so. The central core of the steam devil can be clear, in the same sense that the centre of a dust devil is clear of dust. The core is around 10% of the width of the rotating column.

In some situations like over bigger lakes, steam devils can become much larger. Steam devils ca be about 50m to 200m in diameter, essentially vertical, and up to 500m high. The general shape is like a small waterspout but they should not be considered related. Steam devils rotate with a cyclonic direction of motion, but not very fast or powerfully, usually just a few rotations per minute, and sometimes apparently not at all. There is usually a well-defined inner part of the rotating column of steam and a more ragged outer part from which clumps of steam often detach. The sky above the steam devils may be clear, or there may be cumulus clouds present. In some cases the steam devils may rise directly into the cumulus, in these cases the cumulus may actually be caused by the steam devils - see below. Steam devils are a rare and short-lived phenomenon, typically surviving no more than three or four minutes.

Steam devils can become detached from their base and be blown downstream by the wind. On small bodies of water such as hot springs this can mean that the steam devil ends up over land away from the water altogether. Such steam devils continue to rotate even after they have become detached from the source of heat, but will soon dissipate.

Very small steam devils may have a poorly defined column and no identifiable clear inner core. Such vortices are more properly called steam whirls by analogy with the dust whirls of land.

Steam fog (arctic sea smoke) and steam devils, Lake Michigan, 1971. The air temperature was -21°C (-6°F) and the water temperature 0.5°C (33°F) - a difference of 22°C. Source: Wikicommons

Formation

A precondition for the formation of steam devils is the presence of a layer of moist air on the water with the misty air (called steam fog) being drawn upwards into fog streamers (non-rotating columns of steam fog). For this to happen the body of water must be unfrozen, and thus relatively warm, and there must be some wind of cold, dry air to form the fog. The cold air just above the water surface is warmed by the water and is humidified by evaporation. The warmed air begins to rise, and as it does so is cooled adiabatically by the falling pressure causing the water vapour content to condense out into fog streamers.

For steam devils to form the air above the body of water must be very cold. The temperature difference between the water and the air needs to be quite marked, for example 20°C. This sets up extreme instability in the planetary boundary layer. Under these conditions the air rises so energetically that the air flow becomes unstable and vortices start to form. Fog streamers drawn into the vortices render the vortices visible and they then become steam devils.

With strong winds, the steam fog may indicate irregular hexagonal cells in the horizontal plane which are elongated in the direction of the wind. In this honeycomb arrangement, three cells meet at a junction, and it is in these places that the steam devils form. This effect of vortex formation at the vertices of hexagonal cells is an example of vertex vortices.

The layer of cumulus sometimes seen above steam devils during cold air outbreaks on Lake Michigan and elsewhere may not be coincidental. Airborne radar studies during cold air outbreaks on the lake have shown that some steam devils penetrate through the thermal internal boundary layer (below which convective circulation takes place) and may be more significant for thermal mixing than normal convection, transporting moist air vertically above the convection boundary. The resulting large scale view is a layer of arctic steam fog close to the water surface, a layer of cumulus just above the convection boundary and a regular array of steam devils joining the two.

Occurrences

Steam devils are seen on the Great Lakes in early winter. They occur in the Atlantic off the coast of the Carolinas when cold air from the continent blows across the Gulf Stream. Steam devils can occur on small lakes and even over hot springs, but rather more rarely than on large bodies of water. It is also possible for steam devils to form over wet land if the air is cold and the sun is heating the ground.

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