Steam ring. Steam ring being blown from the Bocca Nuova vent of Mount Etna, Sicily. Steam rings are generated when a pulse of steam is ejected from a near-cylindrical volcanic vent. The steam in the centre of the pulse is propelled by thermal forces, whereas the steam at the edge of the ring suffers drag from the surrounding air. This results in an annular rotation, which in the correct initial conditions can become stable over a span of a few minutes. This is directly analogous to blowing smoke rings. Photographed in February 2000. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JUERG ALEAN)
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