Wind Weight

In the 4th November issue of YnY, Andy Rice was talking about wind weights and americas cup. A reader has sent an email in this month explaining that it is not possible, I wondered what other people thought.

Myself I think that there is such a thing as wind weight. Because last year the 29er Worlds were in Silvaplana which is 1850m above sea level were the air is less dense and i remember the first day reading 25knots average my anemometer when only felt about 18knots. Get back to the UK doing some winter training at weymouth (-13 degrees windchill) with anemometer reading 18knots when it felt like 25knots. Or am i just stupid and the wind is always the same?

Also the reason why the anemometer will read the same wind speed for 2 different air densitys is because the air/paritcles in the air is going that speed but the particles are more spread out in the less dense air so there are less particles hitting your sail even though there hitting it at the same speed therefore it feels like less wind?????

what do other people think?

I dont really know but find it quite interesting

At 2000m height the air density is 20% less than at sea level. So you would definitely feel less force in the sails at the same wind speed compared to sea level.

The opposite applies to a lesser extent with cold air. If are hardy enough to sail at 3C, the air is 10% denser than on a hot summer day of 30C. So the same wind speed will feel "heavier" in winter. Lots of people claim to observe this but I'm not really certain a 10% difference in force would be that noticeable. The force exerted by the breeze is proportional to the square of the windspeed, so a 5% change in windspeed causes a 10% change in force (more or less). 5% change in windspeed is the difference between 20 and 21 knots, and how many of us would really be able to tell the difference between a steady 20 and 21 knots?

Wind on a cold day will cause the sails of a boat to provide more force than on a warm one (assuming the atmospheric pressures both days are equal). The confusion lies in both that the difference is not that large, and also in what a cup anemometer is actually measuring.

The force exerted by wind on sails is determined by the kinetic energy of the wind; being proportional to the density of the air, times the square of the wind speed. For any given wind speed denser air has greater kinetic energy, and hence gives a greater force on the sails, and vice versa.

For a boat the differences are however small. Roughly; for a 3 degree change in temperature (or a 10mb change in pressure) the density of air changes by 1%. Take a 20kt wind on a day that is 20degrees C. On a 5 degrees C day (15 degrees colder) the same air would have a density 5% higher. Hence the cold air would exert the same force as the air on the warm day would if its speed is the square route of 1.05 (i.e. 1.025) times higher – :this gives 20.5kts, not a big difference!!

A cup anemometer is trying to measure the speed of the wind, not the force (more correctly the ‘dynamic pressure’) produced by it. Ideally the cups spin at the speed the air flows past them – in reality this does not quite happen due to friction in the anemometer. If you want to measure the force (dybamic pressure) generated by air you need something that works more like an aircraft's air speed indicator.