20 Oct 2013

Hayling Island: a tornado or not?

Being a weatherman, you can imagine my interest when I checked Twitter this morning to see that there were report of a tornado hitting Hayling Island in Hampshire.

There have been reports of around 100 damaged homes, as well as trees and telegraph poles being ripped down in what has clearly been a short, sharp burst of severe weather.

Then came the speculation that it had been a tornado that had caused the damage. However, as is often the case with such occurrences, there has yet to be any pictorial evidence to back this up.

Whilst a burst of very strong winds has clearly caused a significant amount of damage over a small area, was this actually a tornado? I thought I’d take a look at the evidence and see how it stacks up.


Unstable atmosphere

At the moment, the air over the UK is what is known as a returning polar maritime airmass. This type of airmass is renowned for producing lively weather because relatively cold air travelling over warm ocean water is heated from below, causing air to rise rapidly.

It’s this rapid rising of air that forms huge cumulonimbus clouds that lead to nasty thunderstorms – hence the description of an unstable atmosphere.

The radar picture below from the Met Office at 7.30am this morning shows a nasty thunderstorm moving over coastal parts of Hampshire, including Hayling Island, with very intense rainfall – shown by the pink and white colours.


Spinning motion in the atmosphere

In meteorology, the spinning motion (speed and direction) of air at different levels in the atmosphere is described as wind shear.

When low pressure systems have brisk winds and unstable air rotating around them, as you would imagine, the spinning motion in the atmosphere is quite significant.

Given the weather set up today, it is not out of the question that this spinning motion in combination with the nasty thunderstorm that occurred this morning produced a funnel cloud or tornado.

However, I’ve seen one eyewitness account quoted in a Portsmouth News online article that described the event as a big grey wall of wind that didn’t seem to have any rubbish or debris in it. There is no reference to any tornado-like formation.

Was it straight-line wind damage instead?

The eyewitness account I’ve described above made me think that it may have been something called straight-line wind damage instead.

Straight-line winds, sometimes referred to as thundergusts, are very strong winds caused by columns of air being forced towards the earth’s surface from downbursts in thunderstorms.

As the rapidly moving downburst of air hits the ground, it spreads outwards in all directions, giving a wall of sudden strong winds that can reach between 50-80mph. This can also be accompanied by a sudden, sharp change in wind direction when the thunderstorm has moved through.

The Met Office wind observation graph below from today for Thorney Island, which is right next to Hayling Island, shows that between the hours of 7am and 8am, there was not only a sudden change in wind direction (from S to WSW), but also a rapid increase in the strength of wind gusts.


The blue bars show steady wind speed and the green bars gusts. At 8am, the wind gusts were peaking at around 45mph, which would coincide with the reports of damage on Hayling Island at 7.30am.

A sudden change in wind direction may have proved just as problematic as the very strong gusts themselves.

When strong winds blow in one direction, things like trees bend and have reasonable tolerance before being blown over. However, a sudden change in direction of very strong winds can have a levering effect and wrench things from the ground.

So, was it a tornado?

Unless pictorial evidence surfaces, it is hard to say categorically whether or not it was a tornado that hit Hayling Island this morning. As I’ve mentioned, it could have been straight-line wind damage instead.

The Tornado & Storm Research Organisation (TORRO) specialise in monitoring and analysing data and pictures from damage on the ground to assess whether or not a tornado occurred. It will certainly be interesting to see what they conclude.

In the mean time, low pressure is going to be firmly in control of our weather this week, so there’ll be more wind, rain and thunderstorms to come.

If you have any reports or pictures from the Hayling Island area today, please get in touch on Twitter – @liamdutton

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