Steam ring against the sky, after being blown from the Bocca Nuova vent of Mount Etna, a volcano on the Italian island of 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 drag rotates the steam and forms this annular (ring- shaped) vortex that can remain stable for several minutes. This is directly analogous to blowing smoke rings. Photographed in 2000.

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