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.Barrick, p.213Bluestein, p.151Holle (2007), p.9Lyons & Pease, pp.236-237Zurn-Birkhimer et al., p.2431Bluestein differs from other sources in almost every metric describing steam devils, so much so that he might almost be describing a different phenomenon. Bluestein gives the diameter as ; Lyons and Pease have 50 to . Bluestein has the height as up to 20 feet, Lyons and Pease have 1,500 feet. Bluestein states the minimum necessary temperature difference between air and water to be 68°F; Lyons and Pease give a counter-example of 39°F. Bluestein states there is usually a clear sky; MacDougal and Lyons and Pease both provide photographs with cumulus cloud above. Barrick gives small dimensions comparable to Bluestein, but only in relation to steam devils over geyser basins.
