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.

Although observations of steam devils are generally quite rare, hot springs in Yellowstone Park produce them on a daily basis.

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 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.

Rather smaller steam devils can form over small lakes, especially the warm water in the hot springs of geyser basins.

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.

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 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.

[8][note 1] The steam fog tends to form irregular hexagonal cells in the horizontal plane which are elongated in the direction of the wind.

Steam devils can occur on small lakes and even over hot springs, but rather more rarely than on large bodies of water.