Steam devil

A steam devil is a small, weak whirlwind over water (or sometimes wet land) that has drawn fog into the vortex, thus rendering it visible.

They 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. They are a component of sea smoke.

Smaller steam devils and steam whirls can form over geyser basins even in warm weather because of the very high water temperatures. Although observations of steam devils are generally quite rare, hot springs in Yellowstone Park produce them on a daily basis.

Steam devils have only been reported and studied since the 1970s. They are weaker than waterspouts and distinct from them. The latter are more akin to weak tornadoes over water.

Steam devils were first reported by Lyons and Pease in 1972 concerning their observations of Lake Michigan in January 1971. This month was a particularly cold one for Wisconsin (one of the coldest in the 20th century) which, combined with Lake Michigan staying mostly ice-free, produced good conditions for steam devil formation. Lyons and Pease named steam devils by comparison to the dust devils on land to which they have a comparable size and structure. They were also motivated by the need to distinguish steam devils from the much more powerful waterspout whose land equivalent is the tornado. Lyons and Pease wrote their article with the aim of persuading the National Oceanic and Atmospheric Administration to include steam devils in the International Field Year for the Great Lakes which was imminently to occur in 1972-3.

Steam devils are vortices typically about 50 to 200 metres in diameter, essentially vertical, and up to 500 metres 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. Rather 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. 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, and the smaller ones over hot springs dissipating in a matter of seconds.

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Wikipedia