Beaufort Scale definition.

[AndrewB]

I’ve just finished reading a book by Scott Huler “Defining the Wind” (Crown Press, 2004), all about the Beaufort Scale. The book is almost entirely devoted to the definition of the scale. Yet it seemed to me odd that the book makes no distinction in its discussion between wind SPEED, and FORCE, the effect of the wind on objects like waves, smoke and trees. It is the latter which is the basis of the scale - the ranges of wind speed were added later.

Surely the force is dependent not just on wind speed, but also on air temperature and pressure, and possibly humidity? Thus a wind speed of 21 knots, for example, might be F5 on a warm, low pressure day, but F6 on a cold, high pressure day. This is of slight practical relevance, but seems an odd omission in a book specifically on the subject.

Or have I got the wrong end of the stick?

 

[jonathankent]

Maybe I have got completely the wrong end of the stick... but, I have always understood it that each wind force is defined by a range of speeds, so whatever the weather may be if it is 21kts, then it is always 21kts and always a F5.

I would welcome anyone to tell me if I am wrong, but beware it will need a very convincing explanation for me to change my thinking on this - it makes the whole scale useless otherwise.

 

[theforeman]

“Defining the Wind” - must be up there with
the biafran cookbook,
the italian book of war heroes and
women i have screwed by beverley nicols.
how many pages does it run to ?

 

[landaftaf]

beaufort catagorised the wind - no need to say that if you heve just read the book

in the catagories he defined features to be expected with varous wind speeds, or more precisely, within various ranges of wind speeds.

temp/pressure/etc doesnt come into the equasion, and its based on mean wind speeds out at sea, so fetch comes into it as well ........

his designation of phenomina to be expected at various wind speeds remains valid today and is used throughout the globe - yes even the yanks use it /forums/images/graemlins/cool.gif

 

[milltech]

Interesting. Just as an add-on I noted in an email that weatherweb.net consistently quote a speed in knots that appears to me to be a Force higher than they quote. For example:
17 and 18 knots still as Force 4
22 and 23 knots still as Force 5
31 knots still as Force 6 where 28 to 33 should be a near gale Force 7.

When I was last on the water in 2004 I always seemed to find the winds were stronger than I thought they would be, and I wonder if this was the reason why. I have sent them a couple of emails, but no response so far.

 

[Slow_boat]

If I can slip into psuedo science for a moment, it seems that the idea here is that air in a high pressure is more 'dense' than air in a low pressure area, therefore when it is moving it has more 'force' for a given speed.

Rather like the idea that a lump of steel will hit with more force than a lump of cotton wool at the same speed.

Is that what you're getting at, Andrew?

I don't think it works that way but wouldn't like to say why.

 

[landaftaf]

mmm - sry to interrupt

a constant (average) wind speed exerts a force

whether its hot. cold or indifferent makes no difference .... it generates waves - and poor vis with spume at higher levels

thats the interest to sailors .... not icing or how refreshing a blow is on a hot day /forums/images/graemlins/cool.gif

 

[jimi]

I thought windspeed was really just a rule of thumb to estimate the force exerted by the wind. The force depends on both velocity and mass with the mass being dependent on pressure and content.

 

[Slow_boat]

but aren't the differences of air density due to temperature and pressure to small to be of consequence in the real world?

 

[Pye_End]

yes I agree with Jimi.

Seem to remember an article in PBO some years ago looking at the effect of 'winter air' against 'summer air'. Can't remember whether they looked at density due to temerature, humidity or pressure, but the conclusion was that the effect was pretty insignificant. Therefore velocity was seen as a reasonably good guide to force.

 

[landaftaf]

I can see you are a pedant not a proper sailor with common dog [--word removed--] ... yeah ...... temp etc ........ /forums/images/graemlins/laugh.gif dream on /forums/images/graemlins/cool.gif

 

[Tisme]

That's what I thought too.
Whether or not the differences as a result of low/high pressures and low/high humidity accounts to much I have no idea.

 

[jimi]

Take 970 v 1030 7% difference, equivalent to 2knots at 30knots

 

[KREW2]

Is the velocity of the wind not governed by humidity and air pressure? So as these factors change so does the wind speed. It's the same as sea state, a moderate to slight sea state can be flattened by a heavy down pour.

 

[Pye_End]

It is true to say that humidity and pressure will have an effect on windspeed, but we only measure windspeed and the argument is that we should be measureing force. To then measure force you need to know air density.

 

[Lakesailor]

A Northerly F4 can be bloody awful in winter and a Southerly F4 can be idylic in summer. That's all I need to know.

 

[LeonF]

No weather buff, I am lost. So a Force 4 can be more forceful depending on the weather conditions....but if that causes it to accellerate then surely it merely becomes a force 5 not a more forceful force 4 ??? What I can understand is how the fetch or adjacent land can affect the total picture.... but does a localised forecast take this in to account when predicting wind speeds??

 

[jimi]

No a F4 is a F4 but the windspeed of the F4 may vary. Ie in High pressure it will go to F5 at a lower wind speed than in low pressure.

 

[bilbobaggins]

I seem to recall, from reading accounts of the Whitbreads and Challenges, that the wind speed/reefing diagrams used on many of the boats made a correction for 'heavy, denser air' in the Southern Ocean, such that crews would reef/sailchange at lower apparent wind speeds down there.

It seems the sailmakers and the trimmers knew about this problem.

/forums/images/graemlins/wink.gif

 

[Twister_Ken]

I was rather under the impression that Beaufort defined the forces by their physical effects upon objects like men of war (force 6 - strong breeze - single-reefed topsails and top-gallant sail), rather than anemometer readings, coz he didn't have whirling balls, or pitot tubes. So grafting on windspeeds to Beaufort's observations is a later conceit.