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Many years ago (before children) I used to do quite a lot of windsurfing. We sailed all year and went to the Canaries a few times with our boards.
We used to reckon that, for a given windspeed, we could use a bigger sail in the Canaries than we would in the UK as the warmer air was less dense (certainly seems to lack the same kick when a gust came through).
So were we right or is there no real difference in air density between [say] 10c and 25c?
Not much, about 5%. Maybe the wind speed is more consistent so the gusts aren't as powerful in those climes?
I think there is, purely from a cycling perspective. I also think that damper air is heavier and affects drag.
Just worked it out to be a difference of about 5%; though I can't see how this would translate to a noticeable difference in handling characteristics of a sail.
25C: 1.247 kg/m^3
10C: 1.184 kg/m^3
Wiki has a good table of changing air properties:
http://en.wikipedia.org/wiki/Density_of_air
Air at 0 deg. C has a density of 1.292kg/m^3
Air at 35 deg. C has a density of 1.1455kg/m^3
So, the air at 35 degree C has 0.89x the density of air at 0 degrees C.
Not a lot then. Odd, I remember the wind just felt 'softer'. Must be 'cos I is rad innit.
🙂
5% is going to make a noticeable difference when you're on a time trail though I reckon, where every second counts. If wind resistance was proportional to the air density (no idea if it is) then you're looking at 5% difference or about 15-18 watts of power. It could take me all season to train that much extra power! It's similar to the difference that a time trial bike makes over a roadie with TT bars IIRC - in that kind of ball park at least.
But when cycling what you gain in reduced drag you lose in effective lung capacity.
sharkbait - Member
Not a lot then. Odd, I remember the wind just felt 'softer'. Must be 'cos I is rad innit.
11% difference would be noticeable I would have thought.
Density will definitely matter w.r.t wind resistance - the kinetic energy of a body of air will be proportional to its density.
But when cycling what you gain in reduced drag you lose in effective lung capacity
Oooh yes good point.
Anyone know what sort of conditions time triallers prefer?
I windsurf quite a bit in the Canaries, and there are a few reasons you might get away with a bigger sail.
Your mast will be softer in the warm
The wind is clean and constant, fewer gusts
That beautiful Atlantic swell is just so perfect - you can just power up and sheet in. (except in the harbour at Teguise and the bays at Pozo and Sotavento)
You get a lot of time on the water so you're sailing better
The hire centres like lots of outhaul
Dunno the science behind it but I just about always commute home on the bike into a headwind. Anyway winter winds are always much tougher than spring/summer - even if the wind speed is around the same. Best way I can describe it is that winter winds feel hard and in summer it's soft, like it envelopes you rather than battering you.
Damper air is lighter than dry air because of the hydrogen content.