Waterspout intensity

B. Geerts


The Fujita tornado intensity scale is widely used, and it ranges from F0 (weakest) to F5 (most damaging). An equivalent classification for waterspouts does not exist. The most common type of waterspout is a fairweather waterspout, which arises over warm seas, e.g. near Florida. It lasts for only a few minutes. These waterspouts are generally of F0 intensity, with winds of no more than 30 m/s. They typically occur during the developing stage of an airmass thunderstorm, below the main updraft. These thunderstorms are found in weakly sheared environments, so the spin-up must result mainly from vortex stretching and less from vortex tilting.

Tornadic waterspouts (Fig 1), on the other hand, are far less common but more dramatic and associated with severe thunderstorms over sea. They are more intense, although it is not clear whether intensities of F2 or higher occur at all at sea.

Fig 1. A tornadic waterspout spawned by a supercell storm just off Sydney, Australia, on 22 January 2002. (photo courtesy of S. Golden)