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What's in a name?

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Peter Gibbs Peter Gibbs | 13:00 UK time, Friday, 9 September 2011

Distance travelled ~ 647'166'400 km

(Peter Gibbs is a BBC weather forecaster and a regular blogger for 23 Degrees. With all the current Atlantic hurricane activity, here Peter tackles some of the questions flying around as hurricane Katia makes it's way to the UK.)

"Do we get hurricanes in the UK?"

No, it's not possible.

"But what about the 1987 storm, wasn't that a hurricane?"

Well, it did have hurricane force winds, which was why it knocked down 15 million trees, but it wasn't a hurricane.

"How does that work, then?"

I've lost count of the number of times that I've had that conversation during my career, but it's a reasonable question and one that's worth exploring during this very active Atlantic hurricane season.

interactive forecast track cone katia

Katia moving toward the northeast over the open atlantic

hurricane katia 4 day forecast cone and track

Weakening is indicated since the hurricane is already reaching cooler waters and Katia is forecast to become post-tropical in about 36 hours.

Hurricanes are creatures of the tropics, they need the warmth and humidity of tropical seas to develop and survive. The core of the storm consists entirely of warm air and it's the release of latent heat as this air rises and condenses into clouds which gives the hurricane its power. It's a bit like a pan of water coming to the boil as you apply heat from below.

Once formed, a hurricane moves through the surrounding atmosphere like a cork floating down a stream, becoming almost a separate entity. The strongest winds form in the lowest layers of the storm, close to the storm's centre just outside the eye.

Move into temperate latitudes and weather works differently. Extratropical storms (more commonly known as mid-latitude depressions) form over much colder waters and get their energy instead from the contrast between masses of warm and cold air. The bigger the contrast, the stronger the storm. Cold air makes up the core, digging under the warmer air mass and lifting it until it forms clouds and rain along a front.

The storm becomes an integral part of the atmospheric circulation, like an eddy in a river. Strongest winds are found high up, in the form of the jetstream, at around 30,000ft while the strongest surface winds tend to occur at some distance from the storm's centre and are spread out over an elongated area. All very different to a hurricane.

Where it gets messy is when a hurricane heads out of the tropics and into higher, temperate latitudes. It goes through an identity crisis as the supply of warmth from below is cut off and cold air is drawn into the circulation, eventually emerging as an extratropical storm after a fuzzy intermediate stage.

Fortunately, the long sea track ensures that any ex-hurricanes reaching the UK have gone through full transition before they arrive. The different amounts of available energy mean that even the most powerful of extratropical storms would barely make it onto the bottom of the hurricane scale.

So that's how it works

Comments

  • Comment number 1.

    Nothing this sensible will get any notice. I get sick of the number of times even the BBC show the first part of Michael Fish's infamous 1987 'we won't have a hurricane' comment just before he says 'but we will experience extremely strong winds'

  • Comment number 2.

    Yes 'extremely strong winds' in fact we had 'hurricane force winds'. I remember it well as I was on board a yacht that night at Faversham Kent. Trust me, in such circumstances the distinction between a hurricane and hurricane force winds is academic.

  • Comment number 3.

    #2 I've been stuck in the North Sea in a 30,000 ton ferry for three days that was tossed around like a cork, eventually breaking a prop shaft.... I hate the sea so you have my sympathies. That was a force 12. There isn't a Force 13 because Admiral Beaufort didn't believe any ship could survive more than a 12 which says a lot. I don't think many Brits appreciate EXACTLY what the angry end of a force 12 feels like. Shallow water (like the North Sea or Channel) makes it even worse.

    I feel really sorry for Fish though... he predicted extremely bad weather (which as it turns out was even worse than the prediction but only due to a very slight change in the track of the storm... it was believed it would cross Northern France, instead it changed track by a few degrees and hammered straight up the channel) but the public choose to believe he predicted a nice calm day.

  • Comment number 4.

    Hallelujah. I too have gotten fed up with people talking about the 'Hurricane' of 1987 and trying to correct them. I shall post this link wherever I can, though when you see the dailies talking now about the approaching hurricane, you realise that the truth will never get in the way of a good story.

    My worst ? 5 days in a savage depression in the Australian Bight as a navigator on a 13,000 tonne general cargo ship. Wind estimated continuous Force 12 or more (on current scales) with seas frequently in excess of 100 feet - the bridge was 90ft above waterline and when in a trough, the waves were considerably higher than us !!!

    But it wasn't a hurricane. Neither was it an experience I would wish to repeat.

  • Comment number 5.

    Hi Peter I was reading your Explanation about Hurricanes and i was just remembering the famous line from Micheal Fish.. lol the world is changing and the atmosphere is changing and thing are getting strange in the Weather World. my one liner NEVER SAY NEVER... :)

  • Comment number 6.

    Michael Fish had a point, the world changes, the climate changes and so does language use. Like it or not English generally adapts to common usage, therefore "hurricane" may come to mean something different over time. I find the increasing use of the american pronunciation most annoying though.

  • Comment number 7.

    I agree with SeasideSteve: surely the most important information about a weather forecast is to predict and warn us of the effects of the weather, i.e. hurricane force winds, rather than the technicalities of its source, be it a hurricane or extratropical storm.

  • Comment number 8.

    Peter is absolutely right of course but even so, get caught mid Atlantic in 60 knots of breeze and 972mb and I don't care what you call it. You still kind of take notice! I hope nobody gets hurt when this bit of weather goes through.

  • Comment number 9.

    Peter is right - the term "hurricane" (Atlantic) or "typhoon" (Pacific) refers to a specific type of storm, as does the term "Atlantic cyclone/low/depression". The latter may still pack winds of similar strength to a low-end hurricane but for entirely different reasons - not that such things are much consolation if you suffer from seasickness!

    The worst Atlantic systems do not compare remotely to high-end hurricanes, but are still not to be taken lightly. Ex-Katia certainly likely to cause some disruption Sunday night into Monday.

    Cheers - John

  • Comment number 10.

    Interesting to hear the 'salty dog' tales from the high seas. Never experienced Force 12 at sea myself, but force 10 in the Roaring Forties on the way to the Antarctic was enough.

    Katia slowed down on its way to us, spending longer over cold waters so has lost more energy. Still a potent storm as it arrives on Monday, but more like a 'normal' autumn depression, albeit rather early.

  • Comment number 11.

    Interesting to read that the mechanisms that make a hurricane a hurricane cannot function in our cooler waters, but I did read somewhere that Hurricane Debbie (in 1961) probably still met Hurricane criteria when it crossed the Irish coast - with winds of over 160 kph. I also wonder whether the famous 'Great Storm', described by Daniel Defoe, in November 1703 might have been one - it destroyed a huge number of houses and killed many hundreds. Anyhow, whether or not we can get true hurricanes areas off our west coasts can certainly experience hurricane force winds (Force 12) from time to time

  • Comment number 12.

    I think we are all in for a bit of a blow over the next few days , I urge everyone to tie down anything that may fly about and cause injury. We are also overdue a solar storm too !

  • Comment number 13.

    I will show this to my husband when he gets home - no matter how many times I try to explain to him he won't believe me 'Mr. Fish's Storm' was not a hurricane!
    He still insists hurricane force winds = hurricane - and he's from a seaside community so he should know the difference.

  • Comment number 14.

    Fascinating stories - thanks to those esp re sea experiences! Back in the late 60's - I was a Marconi Radio Officer - at sea for 6 years voyaging worldwide. The worst storm I ever experienced took place less than 100 miles or so SW of Lands End. We had hurricane force winds and waves averaging 45 feet, peaking at over 60 feet! This was not due to any depression directly. We were caught in a very high pressure gradient. This means that a regular depression was approaching from the west and when it came up against a High pressure area to the East the isobars can get very close together - hence the high winds and seas we experienced. So hurricane force winds but definitely no hurricane. Some people may think the difference is academic - but Peter has to reply in this technical way because he is the expert meteorologist. You wouldn't thank him to over simplify his answer.....?
    I've been near hurricanes since then (never right in one TG!) but this was the worst I've ever experienced. And like our friend above - I wouldn't want to repeat it!

 

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