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 arctic steam fog) being drawn upwards into fog streamers. 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 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, and a fairly brisk (over 25 mph) wind of dry air needs to be blowing across the surface of the water. The temperature difference between the water and the air needs to be quite marked; the steam devils in figure 1 were forming with an air temperature of -21°C (-6°F) and a water temperature of 0.5°C (33°F) - a difference of 22°C (39°F). 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.
The steam fog tends to form irregular hexagonal cells in the horizontal plane which are elongated in the direction of the wind. In this honeycomb arrangement, three cells will meet at a junction, and it is in these places that the steam devils form. This effect is an example of vertex vortices.
The layer of cumulus 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.
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