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Wind Weight

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Stefan Lloyd View Drop Down
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Topic: Wind Weight
    Posted: 28 Nov 05 at 1:40pm

In summer, thermal mixing causes the breeze to pick up during the day (I don't mean sea-breeze, I mean inland). That's why on a fine summer day it is often calm first thing and in the evening. In some circumstances you can get a strong temparature inversion within a few metres of the ground. Often you will see mist trapped close to the ground by this effect.

However if there is a strong breeze driven by a pressure system, turbulence causes the mixing to take place anyway, so you don't get much diurnal variation.

I'm not really certain that this will cause wind weight differences between summer and winter though.

Personally, I think it is more psychological and physiological. It is just harder to sail in the same weight of breeze in winter. You are encumbered with extra clothing and your muscles don't work nearly as well when cold. I was very aware of the latter when I used to windsurf in the winter.

 

 

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Ian29937 View Drop Down
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Post Options Post Options   Quote Ian29937 Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 05 at 12:50pm

How does the temperature/altitude/humidity impact the wind gradient ie the difference in wind speed between the top and bottom of the mast? 

I always had a nagging feeling that the different weight of wind which some people report might be due to the different twist profile required in the rig as the gradient profile changed.

Any thoughts from the meteorologists on the forum to confirm or deny?

Ian

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Scooby_simon View Drop Down
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 05 at 11:55am
Originally posted by Jon Emmett

The important part is how it affects the sailing... So is it more than measuring the wind speed, then using the loos gadge to get the correct tension. Perhaps you need a wind density correction factor (percentage wind weight???) when sailing in extreme hot/cold conditions!


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Jon Emmett View Drop Down
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Post Options Post Options   Quote Jon Emmett Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 05 at 8:19am
The important part is how it affects the sailing... So is it more than measuring the wind speed, then using the loos gadge to get the correct tension. Perhaps you need a wind density correction factor (percentage wind weight???) when sailing in extreme hot/cold conditions!
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 27 Nov 05 at 11:58pm
I'm impressed by you guys.
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 05 at 4:10pm
Originally posted by Entyplod

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.



I knew that bit.  Just did not want to complicate it any more for now.....


 
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Entyplod View Drop Down
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Post Options Post Options   Quote Entyplod Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 05 at 4:08pm

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.

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Scooby_simon View Drop Down
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 1:29pm
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.

 

 So we were agreeing with each other

Excellent.

 

What I was trying to say is that the few degrees of them and thus density of the air was not really worrying about.

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Post Options Post Options   Quote yellowhammer Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 12:56pm

Aircraft cruise high (low dencity) and cold ... gives higher speed at design Mach No. for wing and reduced drag. Not sure it reads across to a 0.03 Mk No (20 kts) at sea level.

Dencer air = more molecules in contact with sail = more drag, another factor to consider.

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Stefan Lloyd View Drop Down
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 12:54pm

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.



Edited by Stefan Lloyd
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