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Old 08-12-2007, 19:52   #1
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Wind strength/temp

I got to thinking today (yeah I do that once in a while) about wind strength and it's affect with temperature. Being that cold air is more dense, would it be true in saying that a cold wind at say 10c 20kts would have more force than a warm wind at 20c 20Kts? It's just that my thinking was in relation to being in a blow in the winter and thinking it was damn aweful, and being in a blow in the summer that was similar wind strength yet it didn't seem be so bad. Now it may be that other factors were a major influence, such as tide and location. So an thoughts or answers on this???
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Old 08-12-2007, 20:20   #2
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Originally Posted by Alan Wheeler View Post
Being that cold air is more dense, would it be true in saying that a cold wind at say 10c 20kts would have more force than a warm wind at 20c 20Kts?
Yes.........
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Old 08-12-2007, 20:24   #3
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Trouble sleeping at night Alan? Seriously, interesting question!
I thought about this for a moment, and then decided that wind strength is a factor of Atmospheric pressure gradient and that differential should remain constant, regardless of air temperature.
I think if you were to create equal pressure differentials in a given area, at different temperatures then measure wind velocity, they would be the same.
I think the winter winds subjectively feel stronger for 2 reasons:
1. Wind chill factor.
2. Isobars are generally closer, with occlusions in the mix.
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Old 08-12-2007, 20:39   #4
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Cold air is denser. So at the same velocity it applies more pressure to you and your sails.
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Old 08-12-2007, 20:40   #5
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Originally Posted by Pelagic View Post
Trouble sleeping at night Alan? Seriously, interesting question!
I thought about this for a moment, and then decided that wind strength is a factor of Atmospheric pressure gradient and that differential should remain constant, regardless of air temperature.
I think if you were to create equal pressure differentials in a given area, at different temperatures then measure wind velocity, they would be the same.
I think the winter winds subjectively feel stronger for 2 reasons:
1. Wind chill factor.
2. Isobars are generally closer, with occlusions in the mix.
Nope............



Lower air density means there are fewer or lighter molecules (per air volume) to flow over the lifting surface. Air density is determined by three factors.
  1. Pressure -- The weight or force per area exerted by a column of air. Common pressure units are pounds per square inch (psi), inches of mercury (Hg) or millibars. Standard atmospheric pressure at sealevel = 29.92 inches of Hg.
  2. Temperature -- As air is warmed, it expands and thus has a lower density. Hot air rises, which is what keeps hot-air balloons up, and cold air sinks. Standard atmospheric temperature at sealevel = 59.0F.
  3. Humidity -- Water vapor molecules (H2O) are lighter than are nitrogen N2 and oxygen O2 molecules that make up most of the atmosphere. Humid air has a higher proportion of light water molecules than does dry air. The result is that wet air is lighter than dry air. This humidty effect is fairly small.
The combination of these three factors determines the air density and thus lifting power for a given wind speed
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Old 08-12-2007, 20:43   #6
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Cold air is denser, so yeah. Whether or not the difference is noticeable is probably the real debate.

Denser air would have a higher specific heat and a higher thermal conductivity..if that has anything to do with how air "feels".

Someone would have to come up with a table in grams per cubic meter at a given pressure and at a given temperature to measure the airs density. I'm sure such a table exists.

My gut feeling is there is probably not a whole hell of a lot of difference between a warm day and a cold day...nothing one would notice. I think the difference is probably psychological because a cold wind just seems nastier than a warm wind.

Edit: I found this table

Air Properties

The table shows the air density to be 1.293 Kg/ cubic meter at 32 farenheit and 1.205 Kg per cubic meter at 68 farenheit..for a .088 difference or a 8.8% difference.

Wow....perhaps there is a noticeable difference? Would 8.8% be noticeable?..what if the temp were 50% higher at 86 degrees?...that would be a 13.2% difference in density from an air temperature of 32 degrees. Since Boyles Law (how temperature, volume and pressure relate) is a linear function, air density applies as well.

So since there is a 13.2 percent difference in air temperature between freezing and 86 degrees then is there a noticable difference?....yeah probaly. It would be like adding 13.2 percent more sail area to the rig...right?
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Old 08-12-2007, 20:54   #7
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Interesting question.
The short answer is yes, and thats not making allowance for all the other variables like humidity, dew point and as someone said barometric preasure.

So, if its just the temp difference, then yes, there will be a difference in PSI against the sails. At 0 deg C the presure is 0.081 lb/ft3 But at 40 deg c it is 0.071 lb/ft3
The obvious interesting thing is that the presure difference is in numbers I don't understand but would think would be so slight you and your sails wont notice a difference.

These figues came from a chart I googled in a site called Engeneering toolbox These folks sound like true geeks!

Note also I took the zero PSI column but I dont understand it all
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Old 08-12-2007, 21:30   #8
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Originally Posted by David M View Post
Cold air is denser, so yeah. Whether or not the difference is noticeable is probably the real debate.

So since there is a 13.2 percent difference in air temperature between freezing and 86 degrees then is there a noticable difference?....yeah probaly. It would be like adding 13.2 percent more sail area to the rig...right?
I think yes.
I cannot find what I want to show it.
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Old 08-12-2007, 21:46   #9
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Maybe one could find the answer here;

Meteorology - Wikipedia, the free encyclopedia
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Old 09-12-2007, 00:05   #10
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Thanks guy's. The result of your answers is they have caused me to think more.
David M I loved your thinking. The way you placed your thoughts in words as you went. Your math examples make perfect sense. I should have considered that approach from the begining.
Quote:
It would be like adding 13.2 percent more sail area to the rig...
Yep absolutely. It's so obviouse now. In fact, another way to look at this is the formula for Force which is Mass x Acceleration. So if we take the mass of air at 32F or 0C being 1.293 x 40Km
It will be 1.293x11.11=14.37Kg/CM. That same equation gives 13.39Kg/CM for 68F (20C). Over 1Kg more force per cubic meter of air. That may not sound much, but actually it is one heck of a lot of force pushing into your sails, or more to my original thought, Pushing up those waves.
Thanks gang, very interesting.
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Old 09-12-2007, 00:12   #11
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Playing football as a kid, you learn that a cold football hurts a hell of a lot more than a warm one. Part of it (not all of it, just part) is because the air in the ball (which is already under a great deal of pressure has more oomph.
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Old 09-12-2007, 08:45   #12
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Playing football as a kid, you learn that a cold football hurts a hell of a lot more than a warm one. Part of it (not all of it, just part) is because the air in the ball (which is already under a great deal of pressure has more oomph.

We should just let that one slide.

Since there is less pressure in the football when it is cold.................
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Old 09-12-2007, 08:56   #13
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Pushing up those waves.
Thanks gang, very interesting.
There was an example of this effect in Hernando county in 1993.
It was a storm and so what, at that wind speed and all there may be a foot or two of extra water pushed up the Weeki Wachee river, no big deal.
After it was over it was named the "No Name Storm" because it should have been named but was not.

Well what the forecasters and all forgot to take into account was the fact that the air temp was also dropping fast (and winds increasing as it came ashore. An unusual occurrence with that type of circulation at that time of year etc.

The result was a tide of over six feet above high, lots of homes/trailers flooded, some cold folks on roofs for a while, cars (and boats) sunk etc.

Cold air pushes things harder whether it is water (waves), sails, or just the ears.

I am sure that some that have sailed if FL and also Maine (Oregon?) could chime in to tell us if it is noticed.
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Old 09-12-2007, 10:09   #14
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Cold air sailing has way more punch then warm air sailing. It especially feels that way when its snowing.
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Old 09-12-2007, 10:34   #15
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We should just let that one slide.

Since there is less pressure in the football when it is cold.................

So a cold football is supposed to have less density than a warm one? Sure as hell didn't feel that way. Maybe the leather was what made it feel like a brick, but cold footballs = painful.
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