[luck66]

This is likely off topic. But I could find no better place to for it.
I am getting ready to change my sails from the winter sails to the summer sails. The reason is the winter sails are old and slow. Over the years I've noticed that the winter wind and the summer wind are different. Aside from being cold and wet, the winter wind seems to be stronger for the same knots. A 20 knt wind is still 20, but in winter the 20 knts seems to have more punch. I think this effect may have something to do with temperature and the number of molecules being pushed around. Any thoughts?

 

[Donna_F]

Re: Wind and Sails

Luck, I'm going to move this into its own thread. It is a good discussion starter and I don't want it to get buried.

 

[Ajax_MD]

Cold air is denser than warm air, so essentially more mass is pressing on your sails so yes, the punch is greater.

It's the equivalent of being hit with a 5 lb. mass at 20 kts vs being hit by a 2 lb. mass at 20 knots.

Automobile engines meter fuel differently during cold weather because the denser air changes the fuel/air ratio, as another example.

 

[killarney_sailor]

I suspect it is like the difference between being hit by a 5 lb mass and and a 5.1 lb mass. I don't remember my details of Boyle's Law if that is the correct on, but the pressures are different, but not that different with the temps we are talking about.

 

[Minnesail]

I think the volume of a gas is linear with temperature when measured in degrees Kelvin. The Kelvin to Celsius conversion is 273º (thank you Wikipedia).

So if the temperature went from 273ºK (freezing) to 303ºK (a warm sunny afternoon) we do 273/303 = 0.9, so the air would only be 90% as as dense as it was at freezing.

Humidity might have more to do with it. Humid air is less dense than dry air (even though it doesn't feel that way) and warm summer air is going to be a lot more humid than dry winter air.

 

[Stumble]

The density of air doesn't change that much with temprature.

Temp.....density (lbs/foot^3) Air - Temperature, Pressure and Density
30.........0.081
40.........0.080
50.........0.078
60.........0.076
70.........0.075
80.........0.074
90.........0.072
100.......0.071

My guess is that the difference is in your head more than in reality. Any difference here is probably equalled out by the lower absolute humidity of the colder air.