A water spout is a tight vortex made visible by cloud and spray within it, extending dozens or hundreds of metres from a water surface. Sailboats and small motorboats have been capsized by water spouts. They are of two kinds, one of which is the marine analogue of a dust-devil, growing up from a warm sea-surface under a clear sky. Such water spouts occur when the cold air behind a cold front passes over a warm sea, for instance.
The second type of water spout is the maritime counterpart of a tornado. It forms under a convective cloud in the same way as a tornado, but it is less intense and shorter lived. One observed in Port Phillip Bay near Melbourne on 11 April 1994 grew down from a thundercloud when the water surface was at about 19°C and the air at 8°C, so that there was considerable instability. The waterspout reached 300 m high and 30 m in diameter, and lasted for 10 minutes or so.
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