Waterspout Was Seen When the Bayesian Yacht Sank in Sicily. What Is It?

What caused the sinking on Monday of a sailing yacht carrying the British billionaire Mike Lynch and 21 other people off the coast of Sicily is still unknown. But some attention has focused on observations by witnesses, who described seeing a small tornado-like column known as a waterspout forming over the water during an abrupt and violent storm as the vessel sank.

Fifteen passengers on the 180-foot yacht, the Bayesian, escaped on a raft before being rescued by a neighboring cruise ship. The body of the ship’s cook was recovered on Monday and six people remain unaccounted for, including Mr. Lynch and his daughter Hannah, according to officials with Sicily’s civil protection agency.

Prosecutors in the nearby city of Termini Imerese have opened an inquiry into the cause of the sinking.

Here is what to know about waterspouts, a surprisingly common weather phenomenon that may have helped sink the luxury yacht.

What are waterspouts?

Waterspouts are columns of spinning air and moisture — similar to tornadoes over water, according to the National Weather Service.

While some form in fair weather, and are aptly called fair weather waterspouts, another more dangerous variety called tornadic waterspouts develops downward from a thunderstorm. These tornadic waterspouts can either form as regular tornadoes over land and move out to sea, or form in a storm already over a large body of water, according to the National Oceanic and Atmospheric Association.