What is a waterspout and did it cause the Sicily yacht tragedy?

At least one person died and six others – including a software mogul nicknamed ‘Britain’s Bill Gates’ – were missing after a yacht capsized in Italy.

The British-flagged Bayesian had been anchored about a half-mile off Porticello, about 12 miles east of the Sicilian capital, Palermo, when a storm shook the coast.

Fifteen people managed to escape, with the body of the ship’s cook recovered.

Those unaccounted for include: tech titan Mike Lynch and his daughter Hannah; Jonathan Bloomer, chairman of Morgan Stanley International, and his wife Judy; and Christopher Morvillo, a lawyer at Clifford Chance, and his wife, Neda, a jewellery designer.

Witnesses say that they saw a ‘waterspout’ tearing through the seas at the time of the sinking, which happened at about 5am.

But what exactly is this ocean phenomenon and how likely is it that one could have sunk the Bayesian?

What is a waterspout?

You can think of a waterspout as a small tornado tearing through the water.

Rather than solely wind, waterspouts are whirling columns of air and water mist.

Or, as Dr Jian-Guo Li, a senior modelling scientist for the Met Office, puts it to the Metro.co.uk, they are an ‘atmospheric tornado happening over water surface’.

There are two types of waterspouts. Tornadic start as tornadoes on land and move to water, sucking up the moisture like a vacuum cleaner.

Then there’s the more common fair weather. These form during calmer conditions – so not a storm, for example – and stretch out from the water towards the clouds.

Most only last a few minutes – 20 minutes at a push and move at about 20 to 30km an hour. Slightly more intimidating, however, is their size, often towering at several hundred feet and can whip up wind up to 150mph strong.

What causes a waterspout?

They’re caused by the interaction of very cold land air sweeping over warmer ocean air, so are common during choppy weather like storms.

Waterspouts can start off as funnel clouds, the Met Office says, with strong wind stretching and twisting a cloud down without reaching the surface. Think of a rope dangling off the ceiling.

‘Waterspouts form beneath thunderstorm clouds, so need the same ingredients as a thunderstorm,’ says Dr Pete Inness, an Undergraduate Programme Director at the University of Reading’s Department of Meteorology.

‘Heat and humidity in the lower atmosphere are the two main requirements, and over the Mediterranean in late summer and autumn there is plenty of both.

‘Changes in wind direction with height are also needed to set up the rotation of air within the waterspout.’

Are waterspouts rare?

Waterspouts are fairly more common than their land-dwelling cousin, the tornado, and tend to be milder, too.

These eerie columns are most likely to be found in tropical and sub-tropical seas – especially the Mediterranean – but can be seen in the UK from time to time.

Are waterspouts anything to worry about?

If you ask Inness, not really. Most waterspouts don’t have much damage potential – plus, out at sea, there’s a fair few less buildings that can be destroyed.

‘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,’ he says.

Jian-Guo says the problems waterspouts pose are the same as tornadoes, really.

‘You may have seen many American tornado reports, which showed how cars and house roofs are sucked into the air and scattered around like toys,’ he says.

‘When it forms over the water surface, it pumps the water into the air, resulting in the waterspouts. When a boat or yacht happened to be in its centre, it could be lifted and dropped aside.’

Could a waterspout have sunk the Bayesian?

It’s hard to say for sure, both experts stress. Officials are still piecing together exactly what happened in the early hours of Monday that led to the luxury yacht being flipped over.

Prosecutors in Termini Imerese, a city east of where the yacht sank, have been charged with opening a formal investigation.

What is known is that a violent storm rocked Palermo overnight, one predicted by the European Centre for Medium-Range Weather Forecasts.

As ‘brief’ was it was, Italian Air Force’s Center for Aerospace Meteorology and Climatology said, it was ‘intense’ and involved strong gusts of wind and lightning flashes.

Weather reports from Palermo airport, 12 miles west of the yacht, reported gusts of 40mph.

‘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,’ Inness says.

Stressing he is speculating, Iness says that the crew and passengers likely would have had little time to react.

‘Within a waterspout things happen very quickly,’ he says. ‘The strong wind sets in very quickly and changes direction rapidly, within a few seconds or a minute or so.’

The 56-metre-long Bayesian once had the world’s tallest mast at 75 metres, according to the database YachtCharterFleet.

This aluminium mast may have played a role in the sinking, however, with it snapping as the hefty boat tipped over.

‘Rapid changes in wind speed and direction with height in a waterspout could mean that low down the mast the wind was pushing it in one direction, whereas higher up the wind may have been pushing it in a different direction,’ Iness says, stressing he is only speculating.

‘Certainly, any rapid changes in wind direction within the waterspout could cause the boat to rock violently.’

What is the deadliest waterspout?

A tornadic waterspout erupted in Grand Harbour in Valletta, the capital of Malta, in September in the 1550s (the dates are disputed).

Some 600 knights, soldiers and slaves were drowned, according to The Gallery of Natural Phenomena. Four Maltese navy ships were flipped over.

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