Weather expert answers key questions about waterspouts

Dr Peter Inness, meteorologist at the University of Reading, answers key questions about waterspouts following the fatal sinking of a superyacht in Sicily.  

• What is a waterspout?

“A waterspout is a narrow column of rotating air below a thunderstorm that occurs over water. They are part of the same weather “family” as tornadoes. Like tornadoes, many waterspouts are fairly inconsequential, with a few seconds of strong, gusty winds before they move on or dissipate. But some can be much stronger. Because they occur over water the damage is usually more limited than a tornado over land.”

• How do they form?

“Waterspouts form beneath thunderstorm clouds, so need the same ingredients as a thunderstorm. Heat and humidity in the lower atmosphere are the two main requirements, and over the Mediterranean in late summer and autumn, there are plenty of both. Changes in wind direction with height are also needed to set up the rotation of air within the waterspout.”

• How common are waterspouts?

“Waterspouts may be more common than tornadoes. Records are more patchy than for tornadoes though as many waterspouts go unobserved because they are over the sea.  The Mediterranean is possibly one of the places where waterspouts are most likely around the world due to the warm ocean surface and a climate that is very susceptible to thunderstorms throughout the summer and autumn. Many of the scientific studies of waterspouts have been located in the Mediterranean.

“A study by scientists from the University of Barcelona*, looking at waterspouts around the Balearic Islands, found that they are more likely when the sea surface is very warm. Currently the sea surface in the region of Sicily is about 2.5 to 3 degrees C warmer than the 1990-2020 average.”

*Vega, Moreno-Garcia and Guzman, Natural Hazards, 2022

• What damage can waterspouts cause?

“The majority of waterspouts are quite weak, resulting in a short burst of gusty winds, which although momentarily quite strong cause little damage because they are over the sea. But at the more intense end of the scale, winds of above 100 km per hour are possible, although actual wind measurements from inside waterspouts are very rare indeed. Winds of this strength coinciding with the location of a boat are capable of causing damage or capsize, especially because the wind direction varies very rapidly within a waterspout which could cause a boat to rock violently.”