I'm working at indexing the conditions for local observations for temperature, bar. pressure, altitude and humidity.
Over a course of say 45degrees F to 95 F, all other things equal, you'd need an 8.0 in hot weather to deliver the same power as a 7.0 in cold weather. Fairly significant (or why you can't plane in the summer time).
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Interesting. Can you elaborate a bit for the fluid-dynamics challenged ?
Albert
Air gets "thicker" (more dense) as it gets colder. Therefore, at a given windspeed, the sail can generate more power. An analogy is compare the difference in density in air and water. A little fin in the water can counteract the force of a large sail because it is acting in a denser medium.
Sailors at altitude (say Colorado lake) have to rig big even if it's blowing like crazy.
see this rec.windsurfing thread for a better explanation and some of the maths.
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&threadm=pbzR7.1578%24KG5.1234139252%40newssvr12.news.prodigy.com&rnum=1&prev=/groups%3Fq%3Dwhen%2Bis%2B%2Ba%2Bknot%2Bnot%2Ba%2Bknot%26hl%3Den%26lr%3D%26ie%3DUTF-8%26selm%3DpbzR7.1578%2524KG5.1234139252%2540newssvr12.news.prodigy.com%26rnum%3D1