Beafort and gusts

[Ubergeekian]

Sorry to disagree with you old chap, but yes they do! See the word "mean" in the top row.

Oh b*gg*r.

Conveniently, this answers your question as to how they do it - arithmetic mean, as distinct from any of the other averages (e.g. median or mode).

Remember those lessons on statistics?

Even then, what period do they take the mean over? And do they apply any weighting - something like a ten minute exponentially weighted moving average would seem appropriate here.

I still can't locate the definitive source, but here are some snippets from "RYA Weather Handbook"

"Wind speed and direction in a forecast refer to an average wind speed over a 5 or 10 minute period.... On an average day, if there is such a thing, we can expect gusts to be as third as much again as the forecast".

Hmm. 5 or 10 minute?

"...forecast...gale warnings issued if the winds are expected to exceed Force 8 (or if gusts are expected to reach Force 9)"

What's their definition of a gust? Is there a minimum or maximum duration?

Quibbling aside, I find the idea of "F4 gusting F6" perfectly comprehensible and useful. When we came back from Iona a month or two ago we had bands of cloud rolling across us at 30 minute intervals. The leading edge of each one brought ten minutes of increased wind - I'd have described it a F3 gusting F6.

 

[tmh900]

Quibbling aside, I find the idea of "F4 gusting F6" perfectly comprehensible and useful. When we came back from Iona a month or two ago we had bands of cloud rolling across us at 30 minute intervals. The leading edge of each one brought ten minutes of increased wind - I'd have described it a F3 gusting F6.

I'm begining to doubt myself now after reading again the met office definition. I've not seen the knot ranges for each force referred to as "limits" before, and a separate column for the mean value which is between the limits....and no mention of allowance for gusts :

http://www.metoffice.gov.uk/weather/marine/guide/beaufortscale.html

 

[lw395]

Sailing in the Chimet area, I reckon the biggest gusts never hit the anemometer anyway.

 

[guernseyman]

I'm begining to doubt myself now after reading again the met office definition. I've not seen the knot ranges for each force referred to as "limits" before, and a separate column for the mean value which is between the limits....and no mention of allowance for gusts :

If I understand you, you may be reading too much into the word "limits". The limits would seem to me to be simply the range of average wind speeds (including gusts) that are categorised in that Beaufort wind force.

Although there is experimental evidence to suggest that there is a correlation between average wind speed and maximum gust, the correlation is obviously not a matter of common general knowledge even amongst yachties. Ubergeekian is right, there is useful extra information in the gust speed.

Indeed the Met Office do supply such information; see here: http://www.bbc.co.uk/weather/coast/coastalforecast/regional_forecast.shtml?11

 

[tmh900]

Ubergenkian is right, there is useful extra information in the gust speed.

Indeed the Met Office do supply such information; see here: http://www.bbc.co.uk/weather/coast/coastalforecast/regional_forecast.shtml?11

thanks for the link. Agree there is useful info in the gust forecast and I'm fine with it in knots as it is there. I was more concerned with other statements both forecast and actual, where the gust is expressed as a force in it's own right - but we appear to be missing the evidence that is not the convention.

 

[Lakesailor]

As the forecasts are a load of bollocks it doesn't really matter.

 

[aquaplane]

Not everyone knows what wind speed in knots or mph relates to a Beaufort force.

Some folk listen to forecasts quoting Fx or Fy and when sailing look around at how big the waves are and how many white horses there are and how much fun they are having and guestimate what it actually is.

If folk know the numbers I suspect they have instruments telling them.

 

[Fantasie 19]

Not everyone knows what wind speed in knots or mph relates to a Beaufort force.

Some folk listen to forecasts quoting Fx or Fy and when sailing look around at how big the waves are and how many white horses there are and how much fun they are having and guestimate what it actually is.

If folk know the numbers I suspect they have instruments telling them.

Phew - I thought I was the only one - I've always worked in Beaufort and whenever someone quotes knots or mph at me I have to go and look it up...

 

[oldharry]

UberG

Sorry to disagree with you old chap, but yes they do! See the word "mean" in the top row.

Conveniently, this answers your question as to how they do it - arithmetic mean, as distinct from any of the other averages (e.g. median or mode).

But that is not the Beaufort Scale. Met Office uses the Beaufort Scale to define an average or mean wind speed over a given period of time.

The Beaufort Scale was invented to quantify the speed the wind was blowing at a time when therew as no way of measuring it to the nearesty knot as we can now. It is simply a crude measure of speed, not of averages , means or anything else. It was the Met Office needing to give an indication of average wind speeds in their forecasts who defined the .mean wind - then expressed it as a Beaufort figure.

So Chimet is quite right: their average wind speed is more accrately expressed as Beaufort figure, as the probability is that it was within the range of a single Beaufort measurement during the sampling period. They also express the peak gust speed within the sample period both in knots and Beaufort. The Beaufort figure referring to an actual wind speed in this instance.

Met Office didnt invent the Beaufort Scale. It was invented by one Admiral Beaufort as a means of expressing wind speeds which could not in those days be measured any more accurately than the ranges given within his scale of 12.

Nowadays we can measure it accurately enough to scale it to 72 units between Bft 0 and 12. Over 12 the Admiral would have been more interested in trying to save his ships from total destruction than trying to assess the actual wind speed!

 

[Ubergeekian]

Met Office didnt invent the Beaufort Scale. It was invented by one Admiral Beaufort as a means of expressing wind speeds which could not in those days be measured any more accurately than the ranges given within his scale of 12.

According to wikipedia it was invented to describe sea state and link it to the effect on ships. Linked wind speeds came very much later.

Incidentally, wind speed could be measured pretty accurately by the early seventeenth century. Galileo did it, as did Torricelli.

 

[Angele]

But that is not the Beaufort Scale. Met Office uses the Beaufort Scale to define an average or mean wind speed over a given period of time.

The Beaufort Scale was invented to quantify the speed the wind was blowing at a time when therew as no way of measuring it to the nearesty knot as we can now. It is simply a crude measure of speed, not of averages , means or anything else. It was the Met Office needing to give an indication of average wind speeds in their forecasts who defined the .mean wind - then expressed it as a Beaufort figure.

So Chimet is quite right: their average wind speed is more accrately expressed as Beaufort figure, as the probability is that it was within the range of a single Beaufort measurement during the sampling period. They also express the peak gust speed within the sample period both in knots and Beaufort. The Beaufort figure referring to an actual wind speed in this instance.

Met Office didnt invent the Beaufort Scale. It was invented by one Admiral Beaufort as a means of expressing wind speeds which could not in those days be measured any more accurately than the ranges given within his scale of 12.

Nowadays we can measure it accurately enough to scale it to 72 units between Bft 0 and 12. Over 12 the Admiral would have been more interested in trying to save his ships from total destruction than trying to assess the actual wind speed!

Of course I know that the Met Office did not invent the Beaufort Scale but, as UberG points out it was really invented by observing wave heights. It was then linked it to the amount of sail that a Man O' War could carry in those conditions.

As LW395 pointed out earlier, a big fighting ship is not really affected by a gust that passes through, whereas a sailing dinghy very definitely is. And wave height is more a function of mean wind speed than it is of gusts.

So, I think Chimet is wrong. The origins of the Scale suggest that the definition of a particular force is more likely to be linked to averages (which are measured over short periods such as 5 minutes and therefore affected by a big sustained gust) than to the max speed.

I'm fine if people say F5 with 30 kt gusts or 20kt gusting 30. But, I think F5 gusting F7 is simply wrong. The waves in those conditions will be F5 waves.

 

[Ubergeekian]

I'm fine if people say F5 with 30 kt gusts or 20kt gusting 30. But, I think F5 gusting F7 is simply wrong. The waves in those conditions will be F5 waves.

Indeed. That's why it's "F5 gusting F7" and not "F7 with lulls of F5"

However, we now also have sea state forecasts ... doesn't that mean we can keep the Beaufort Scale for the wind?

 

[Blue5]

I'm fine if people say F5 with 30 kt gusts or 20kt gusting 30. But, I think F5 gusting F7 is simply wrong. The waves in those conditions will be F5 waves.

Agreed and to quote UG earlier of saying F3 gusting F6, that could actually be at extremes 10kts gusting 21 or 7kts gusting 26, a bit of a difference.

 

[tmh900]

Indeed. That's why it's "F5 gusting F7" and not "F7 with lulls of F5"

I know I can't prove it by referencing an offical text, but I still claim its not correct to say it either of those ways - its one Force or the other, not both.

Not having much luck with Google. I wonder who would 'owns' Beaufort? IMO? UK HO?

Whilst I shall continue the search for the definitive position, I have successfully contacted Lady Pippa Beaufort, the great, great, great, great niece of the good Admiral and she confirmed that great uncle Frances did indeed intend the 'F' number to include gusts.

 

[Bosun Higgs]

Not around Chimet it won't be .. it's reasonably sheltered by the IOW, but the seastate is far more influenced by the current ....

Come on now - round the south coast / Solent, the sea state with an F4 would be more like that of an F2 in many areas of the UK.

 

[franksingleton]

Just happened on this one.

The Met Office defines the wind in synoptic reports as being an average over 10 minutes. Their automatic stations are programmed accordingly, previously the observer would look at the wind record (pen and ink on a revolving drum) and make an estimate.

Ships at sea used to report wind primarily based on sea state and ships motion. Nowadays they are increasingly usimg automatic weather stations on board. Whether these are better or not is a moot point. By looking at sea state, the OOW was, effectively giving a sensible average. Siting an anemometer on a ship is nigh impossible to do satisfactorily.

Gusts are usually defined as lasting some seconds, I forget how many, probably between 2 and 10.

In forecasting gales, the Met Office issues a gale warning if the wind is expected to be a steady F8 or if it may be a F7 with gusts to F9 speeds.

I do not know just what Chimet and the other non-Met Office data buoys etc is reporting.

The Beaufort scale was defined by the great man in terms of how a ship of the line would handle, carry sail etc. In effect, he codified what was becoming general practice and was introducing uniformity.

Previously, the superintendent of the East India Company, Alexander Dalrymple, had issued pictures of sea state to his captains as guidance in reporting wind in their logs.

The sea state equivalents as published in almanacs etc are used, first to help observers at sea and, secondly, to give some guidance on sea state to users when a wind of FX is predicted.

 

[R32Stbrigid]

I cant locate the official answer but I am sure that the mean wind strength is quoted in Beaufort force x with gusts to yy knots. I.e. the gusts are not to be measured / quoted on the Beaufort scale.

 

[Genoa]

Come on now - round the south coast / Solent, the sea state with an F4 would be more like that of an F2 in many areas of the UK.

Not necessarily. Fireball is right about the current in and around the solent. 4-5 knot tides in the solent, near some of the shallows (or the forts narrows), wind against tide WILL cause much worse sea conditions than the same wind offshore for 2-3 hours. The waves will be steep to and high, unlike many more exposed places with a longer fetch where the tidal rate is less. Do not let newbies think that the solent is safe in these conditions!! However, as quickly as the seas rise they subside ... but much can happen in those 3 hours, ask the LB crews!

 

[fireball]

J<snip>I do not know just what Chimet and the other non-Met Office data buoys etc is reporting.
</snip>

From Here
Wind Speed - Anemometer - Sample Every second - Averaging 5 minutes - Reporting Every 5 minutes

Wind Gust - Anemometer - Sample Every second - Taking max of 3-second running average - Reporting Every 5 minutes

 

[Ex-SolentBoy]

From Here
Wind Speed - Anemometer - Sample Every second - Averaging 5 minutes - Reporting Every 5 minutes

Wind Gust - Anemometer - Sample Every second - Taking max of 3-second running average - Reporting Every 5 minutes

..which is very useful data.

At first I wasn't confused. I thought wind speeds were wind speeds, irrespective of the units used to express them. Personally, I prefer something as granular as possible, i.e. a specific number like 23 gusting 29, as opposed to a range like a F4. However, I accepts that others are used to, and like Forces.

Likewise, I would never try to assume the sea state from the wind speed, apart from assuming that it will be at least as bad!