Heavy Wind

[vyv_cox]

Re: No ... its WIND pressure

Yes, I realise that but the pressure is not acting on a vacuum, it's acting on the same stuff at the leeward side of the sail. So you have to subtract more in the case of moist air than you do for dry.

That's a good site you found but I will check the values given. The figure for Force 11 is 660 N/m2, equivalent to only 13.7 lb.ft2. See the much greater numbers that I found in the "Bruce Anchors" thread. I was searching for a couple of hours earlier in the week when trying to calculate anchor warp loadings under maximum conditions, but I didn't find that one. I did find several American sites that give basic data and the coefficients that need to be applied for differing conditions. The UK codes do something similar, my daughter is very familiar with these and she has given me some formulae. I'll check them against the site values.

<hr width=100% size=1>

 

[Aja]

Re: No ... its WIND pressure

....so its all in the mind then? On a sunny day the wind just feels less - or have I missed the point completely?

<hr width=100% size=1>

 

[jimi]

Re: No ... its WIND pressure

Well .. conceptually I can understand air of greater density and weight (albeit at the same air pressure) having a greater effect on the sails at the same wind speed. Although as earlier said the Beaufort scale measures force anyway and not windspeed. Empirically I would say that I have tended to reef at lower windspeeds in the easterlies we had in the winter (assoc with dry high pressure) than in the damper low pressure systems in the summer.

<hr width=100% size=1>

 

[AndrewB]

Re: ah but erm

Nah, Ken's right. Water vapour is NOT air with water droplets in it, and nor is humid air, any more than sweetened tea is tea with lumps of sugar floating around in it.

You're thinking of rain, and that that comes down all right, except in Manchester where it always comes sideways.



<hr width=100% size=1>

 

[Aeolus_IV]

Re: No ... its WIND pressure

How about this: To simplify the question consider dry air only. At STP (Standard temperature and pressure) a fixed volume of gas has a fixed mass (T/P=V remember to use standard units for measure these properties /forums/images/icons/smile.gif). Keeping the volume the same, but reducing the temperature, reduces the pressure, but the mass stays the same. Like wise, increasing the temperature raises the pressure, but agains the mass stays the same. So to answer the original question (in simplified form) "why is a cold F5 heavier than a warm F5?" we can say the following: if on day A the value of T/P is less than the value of T/P on day B, then the wind of day A will feel "heavier". Indeed it is, a cubic meter of air on day A will have more mass than on day B, and so moving at a given speed will contain more enery, so there is more power available to sail with. But this says nothing about which day this happens on (warn or cold) - I presume that day A will be colder than day B as winter winds always feel heavier to me too.

Adding water to the equation makes things more complex. While the pressure and temperature of the air remain above dew point, the water will will not condense into water dropless, and so the above comments should remain valid. Once air saturation reaches 100% and condensation occurs you now have a mixture to two items in the air (one gas, one liquid, but you also get a change in air pressure as well). You would now need to make an estimate of percentage volume of the air which has been repaced by water (at a 1000Kg/cubic Meter since this will be fresh water). There must be a formula for determining the mass of water vapour which can be supported by a fixed volume of air at known temperature and pressure, so it seems likely that it would be possible to make this estimate, and incorperate it into the calculation.

Naturally the trouble with all this is that there are too many variables working together.

Regards, Jeff.



<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Aeolus_IV on 09/05/2003 14:08 (server time).</FONT></P>

 

[tome]

Re: No ... its WIND pressure

I'd go along with this Jimi, the difference between summer and winter winds of the same force is noticeable.

<hr width=100% size=1>

 

[NigeCh]

Anchor warp loadings

I came across this site last year when trying to calculate optimum scope for various conditions. There are about 7 pages including some interesting maths on wind loadings on boats at anchor.

http://www.dulhunty.com/dmp1.htm

 

[NigeCh]

Formulae for moisture content in air

The formulae are complex - You don't really want to know them, but if you do, then I'll type them in - but I'll give a some examples for a standard pressure of 1013.25 hecto-Pascals, ie 101325 N/m^2

1) at 0 C with 100% RH the moisture content per kg of air is 3.8 grams with a specific volume of 0.778 m^3

2) at 10 C with 100% RH the moisture content per kg of air is 7.7 grams with a specific volume of 0.8116 m^3

3) at 20 C with 100% RH the moisture content per kg of air is 14.75 grams with a specific volume of 0.8497 m^3

4) at 30 C with 100% RH the moisture content per kg of air is 27.31 grams with a specific volume of 0.8959 m^3

<hr width=100% size=1>

 

[vyv_cox]

Re: Anchor warp loadings

Excellent, thanks. Best I've seen to date.

Vyv

<hr width=100% size=1>

 

[Aja]

Re: No ... its WIND pressure

Jeff

Yup - that got through!

There was and article - many years ago in one of the mags and I'm sure that it referenced a Scandinavian country where the force of the wind differed greatly between seasons for the same wind speed, and they called it 'Heavy Wind'.

Many thanks for your and everyone else's replies.

Regards

Donald

<hr width=100% size=1>

 

[claymore]

Re: No ... its WIND pressure

Tom, Jimi
Surely, the whole issue of overlapping genoas is the mission critical factor here. On my Leisure 17 (bilge Keel) I used to reef earlier when the wind was coming from the front of the boat. Failure to do so would inevitably lead to moist water droplets being conveyed over the whole of the boat at pressure. This in turn could trigger the very conditions which TCM referred to in an earlier post - the detail of which I do not have at present.
Jim talks of empirically reefing in the dry mainland continental airstreams at a different moment in time to the need to carry out the same actions in the temperate (moist) atlantic conditions brought on by a prevailing south or westerly airstream. Again, your posts are flawed in that at no time do you consider convection.
I do feel that in failing to mention the venturi effect, the very essence of all relations between main and head sails, you are both courting the temptation to ignore the effect on conditions of the seasonal gradient winds, in the medium wind range, experienced close to windward shores in the northern hemisphere.
Discuss.

<hr width=100% size=1>regards
Claymore
/forums/images/icons/smile.gif

 

[Aeolus_IV]

Re: No ... its WIND pressure

Donald,

Just trying to apply a little objectivity into an interesting question.

Using my own argument I've worked out the difference (in dry air conditions of course) between a day at 30 Deg C at a pressure of 1035 Mb and a day at 5 Deg C at 980 Mb. The colder day is 3% heavier than the hotter day. Not enought to be noticable at a subjective level I'd say.

Now (for fun?), I'll see if I can understand Nige's comments. Not likely as I'm not a weather man, and my physics education was some years ago now....

Regards, Jeff.

<hr width=100% size=1>

 

[NigeCh]

Heavy stuff - (Ignore this if you dont like sums)

Units in the calculations: barometric pressure; % saturation; moisture content; specific enthalpy; specific volume; vapour pressure; dew-point; adiabatic saturation temperature; dry bulb temperature; and wet bulb temperature.

It's volume that counts unless volume is standardised and everything else is interpolated to suit a standard one cubic metre of moist air.

I've spent over 30 years working on ithe properties of humid air - It's driven me nuts ... Here are the basic formula for working out whether air gets heavier or lighter.

Volume = ( 82.0567(273.15 t)/(28.966(1013.25 - Pa)/1013.25) ) - (Xa^2 x Aaa Xa(1 - Xa) * 2Aaw (1 -x)^2 * Aww

where:
t is the dry bulb temperature in degrees C;
Xa is the mol-fraction of dry gas, ie 0.62197/(0.62197 g) where g is the moisture content in kg/kg of dry air (Formula for alculating 'g' follows);
Aaa is the 2nd viral coefficient for dry air in M^3/kg;
Aaw is the interaction coefficient for moist air inm^3/kg;
Aww is the 2nd viral coeffficient for water vapour in m^3/kg

You will aslo need to know that moisture content aproximates to

Moisture content in grams (The 'g' above) = 0.624 * Pss / (Pat - 1.004 * Pss)

where:
Pat = barometric pressure in hecto-Pascals;
and Pss is derived from:

Log Pss = 28.59051 - (8.2 * log10(t 273.16)) (2.4804 * 10^-3 * (t 273.16)) - (3142.31/(t 273.16))

where t is the dry bulb temperature in degrees C.

The above equations coupled with some of the formulae in the Dulhunty site (posted elsewhere in this thread) should give the gurus here enough information to demonstate the heaviness of air and its effect on sails - There's a bit of lateral thinking required to connect the various parts.

I was rubbished when I posted a similar thread title here 3 years ago - It's nice to see a new thread with some serious responses to a totally trivial question - What we eyeball on the sea surface state tells us what the wind pressure is.


<hr width=100% size=1>

 

[NigeCh]

Re: Mathematics is a hobby ...




<hr width=100% size=1>

 

[tome]

Re: No ... its WIND pressure

Certainly the slot has an important role to play, but then so did Vivien Leigh in David O'Selznick's Civil War epic Gone With the Wind.

As to convection, I fear there's more hot air entering this debate as it grows ever lengthy. Time to scuttle my butt down to the boat, me thinks.

Forecasts vary for the weekend so if it's Westerly I'm off to St Vaast and if it's Southerly then Weymouth. Either way, it doesn't look like there'll be much weight in the wind.

<hr width=100% size=1>

 

[Seafort]

Re: No ... its WIND pressure

I agree.

But

Does everyone sail on a run?

Venturi effect yes. Most of the time my sails act as aerofoils, pressure differential.

Of course when on a run different air masses (note term) have differing "weights" /forums/images/icons/crazy.gif, but so does the air infront of you.

Dave./forums/images/icons/wink.gif

<hr width=100% size=1>

 

[peterb]

Re: Why is moist air lighter?

It's not the atomic weight that matters, it's the molecular weight. Both nitrogen and oxygen have two atoms per molecule, so their molecular weights are 28 and 32 respectively; since about 80% of the atmosphere is nitrogen the density of air is equivalent to a molecular weight of about 28.8. The molecular weight of water vapour is 18 (16+1+1), so it's density is not much more than half that of air.

<hr width=100% size=1>

 

[Aeolus_IV]

Re: Heavy stuff - (Ignore this if you dont like su

NigeCh,

It looks like you're missing some symbols - I believe I can see where you convert Deg C into Deg K, but the "+" is missing. Or perhaps I am missing something?

Regards, Jeff.

Yes, it is quiet at the moment....

<hr width=100% size=1>

 

[DeeGee]

Re: Heavy stuff - (Ignore this if you dont like sums)

I've spent over 30 years working on ithe properties of humid air - It's driven me nuts ...

Sooo, that's what did it.!!!


<hr width=100% size=1>Black Sugar - the sweetest of all