Heavy Wind

[Aja]

Is there such a thing - other than the scene from 'Blazing Saddles'.

I seem to remember reading some years ago that wind density increased with a dip in temperature - you know the feeling - a F5 beat in a nice warm sunny day is pleasant - a F5 beat when it is overcast or wet is an entirely different thing.

Can the wind pressure (lbs/Ft2) change that much?

Thanks

Donald

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[Twister_Ken]

A Force 5, as described by the good Admiral Beaufort, is not a windspeed, but the weight of wind which has an affect upon the ship and the sea. So a F5 will always be a F5 whatever the wind speed and the air density.

Things do get denser as they get colder (esp. my brain) so cold air will be heavier than warm air, and thus exert a greater pressure on that which it hits. So 20kts in summer will be towards the top end of enjoyable (for ordinary mortals at least), while 20kts of snow-laden winter wind will be distinctly more testing.

In addition, one could presume that 20kts of wind in a high pressure system would be more powerful than 20kts of low pressure air.

No doubt a physicist could quantify how much denser air at 0 degrees is than air at 25 degrees, or at 1030 mb than at 980mb. I can't though.

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[sailbadthesinner]

Re: but what about

the effect that warm air can hold more water vapour than colder air?

basically if the absolute humidty of an air mass is higher then surely its mass its greater and combined with velocity means force is greater?

i am just scrabbling around long lost schoolboy physics here and totally accept i may unutterably incorrect.

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[jimi]

Re: but what about

Sailbad, yiou're as thick as a bad heid on a Setterday morning! Air pressure takes into account the weight of everything in the atmosphere ... including seagulls.

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[vyv_cox]

Yes, it changes considerably. Standards that give strength requirements for buildings in various locations require the application of many factors that include one for temperature. Strength requirements at similar altitudes and regions (coastal, desert, rural, etc) are greater as you progress further north from the equator.

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[jamesjermain]

Re: but what about

Ah! That explains why Sweet Lucy heels so much more when seagulls fly into the sails. I find the answer is to feather the main and set the genoa wing and wing!

Sorry just had a bit of a tern there

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[jimi]

just as well ...

... you were'nt puffin yer pipe stuffed with shag whilst sorting the gooseneck despite being too chicken to remove yer bonnet as yer as bald as a coot!

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[sailbadthesinner]

Re: just as well ...

well you have no reason to crow
ye often make a right tit o yerself and and make us owl with laughter


(but at least you know your met stuff. Air pressure DOH!)

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[sailbadthesinner]

Re: but what about

doh
guess who got an A in geography A level including weather

brain not in gear today

<hr width=100% size=1><font color=red>I can't walk on water, but I do run on Guinness</font color=red>

 

[sailbadthesinner]

Re: but what about

doh
guess who got an A in geography A level including weather

brain not in gear today

<hr width=100% size=1><font color=red>I can't walk on water, but I do run on Guinness</font color=red>

 

[claymore]

Re: just as well ...

Well, I wiz talkin' tae this Black Bird an she reckoned that I wisnae fit on acoont o ah wiz puffin a bit - ma pal Morant jes sed 'Cor' but widnae budgie a wee bit frae his stance despite the fact that ah caud him by ra Glaswidgeon term o Hen

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Claymore
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[jimi]

Re: just as well ...

Och ah bet the bustard wiz raven mad!

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[claymore]

Re: just as well ...

Ah've run oot o birds - Ahm sayin naethin frae heron

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Claymore
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[AndrewB]

Newbie-ish calculation

I'm no expert, but if I understand this, its not too hard to work out a possible range.

As explained for example by <A target="_blank" HREF=http://www.knmi.nl/samenw/hydra/faq/press.html>this site</A>, the pressure exerted by the wind is a function of the air's density, the wind speed and the drag coefficient of the sail or whatever: that is

Pressure = ½ x (density of air) x (wind speed)^2 x (drag coefficient)

where pressure is measured in Newtons per metre^2, density in Kg per metre^3, and wind speed in metres per second.

The density of air is determined by air pressure and temperature (adjusted for humidity) as follows:

Density = Air Pressure /(Adjusted Temperature x 2.87)

where air pressure is measured in millibars and temperature in degrees Kelvin (Absolute).

Thus for the same yacht in the same wind speed, the pressure on the sail is affected by air pressure, temperature and humidity.

Air pressure varies between about 970 - 1030 mb, i.e. about 6% from one extreme to the other. Temperature (when we are sailing anyway) from maybe 270K to 305K, i.e. about 13% from one extreme to the other. Humidity (vapour pressure, in normal ranges) can affect adjusted temperature by about a further 1%.

So combining these together, on an exceptionally damp, low pressure, cold day, the wind pressure at the same speed could be 20% greater than on an exceptionally dry, high pressure, hot day.

 

[Tantalus]

Re: Newbie-ish calculation

I think you all need to consider taking up a hobby. I highly reccommend sailing. : )

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[claymore]

oooooHHH

Sauce

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Claymore
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[AndrewB]

Mathematics is a hobby ...

... but sailing is a way of life.

 

[Twister_Ken]

Oh boy

This takes me back to A'level Physics (failed), and that was a long time ago:

>So combining these together, on an exceptionally damp, low pressure, cold day, the wind pressure at the same speed could be 20% greater than on an exceptionally dry, high pressure, hot day<

Error might have crept in here - surely a high pressure wind will be 'heavier' than a low pressure one?

And - this is what I mean about Physics, I seem to recall that - bizarrely - moist air is less dense than dry air, which is one reason why clouds float. Can anyone more knowledgeable confirm?

So the worst of all worlds would be a cold, dry, high pressure day. Maybe.

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[Twister_Ken]

Re: but what about

>basically if the absolute humidty of an air mass is higher then surely its mass its greater <

I think this may be a-a-f. I seem to remember the simplistic explanation about why clouds float is to do with the fact that moist air is lighter than dry air. Sounds daft though, dunnit?

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[Twister_Ken]

Re: just as well ...

I'm choughed to have come up with another one. And if I warblered on a bit more I could probably think of others. Anyway, crane me out of here, and duck as you do it. I'm not feeling bittern' twisted, just a little cross, Bill. Though maybe I'm going cuckoo. Stop grousing about my puns, and eat more hen, or go fly your kite.

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