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A funnel cloud, similar to a Tornado, which projects below a cloud over water.
Waterspouts are similar to tornadoes in structure but occur over the sea rather than over the land and are not usually associated with a mid level (Mesocyclone) rotation within the parent Cumulus/Cumulonimbus (Cb) cloud. They are believed to be produced along surface airmass convergence boundaries. As the parent cumulus cloud develops in intensity, so the potential for a waterspout increases. Surface spray may be seen at the base of the waterspout yet, until the wind speed in the vortex reaches about 35 kts64.82 km/h
, the waterspout may not be visible. A fully developed waterspout will be quite distinctive with the funnel filled with water droplets.
Waterspouts are less intense and smaller than tornadoes; a waterspout rarely lasts more than 30 minutes, is only 10 meters in diameter and approximately 100 meters deep. Nevertheless, they represent a significant hazard to low flying aircraft, particularly helicopters operating at low level over the sea, because of they are not easy to see unless they are fully developed. They are also rarely a stable vertical feature so need to be given a wide berth.
A Waterspout will subject any aircraft encountering one to severe turbulence which may cause structural damage, injure crew and passengers, or even result in Loss of Control with fatal consequences given the proximity to the surface of the sea at low altitudes.
- Avoidance: Avoid flying under a developing Cumulus or Cumulonimbus (Cb) cloud. Weather/doppler radar may not detect the presence of a waterspout.
- Detection: The surface of the sea, where the waterspout touches the surface, will be disturbed and there will be considerable surface spray.
A pilot on final approach to an island airport sees active piled cumulus clouds (cumulus congestus) ahead, which is confirmed by the weather radar. He sees circular patterns of disturbance on the surface of the water ahead and, suspecting the presence of a waterspout, carries out a missed approach. As he climbs and turns away from the disturbance the aircraft experiences mild turbulence.
Given the narrow diameter of any waterspout, it is likely that the aircraft will have exited the waterspout before the crew have much time to react. In such circumstances basic principles apply:
- First: fly the aeroplane (helicopter)
- Second: maintain a safe altitude and track to avoid obstacles.
- Third: check for damage to the aircraft and occupants.
- Warn other aircraft in the area.
Accidents and Incidents
- On 28th February 2002, an AS322 Super Puma helicopter, located over the North Sea, 70nm northeast of Scatsa in the Shetland Islands, UK, suffered tail rotor and pylon damage when it encountered severe weather generated vortices associated with a waterspout. For further information, including detailed information on waterspout formation, see the full AAIB Incident Report
- Tornadoes in Europe - An Underestimated Threat: article from American Meteorological Society journal, April 2017.