Sep 21, 2011, 4:30 PM
Post #7 of 12
The impression left by the "Wind strength Myth" answer is I feel misleading as at higher temperatures you need a higher velocity to feel the same force, not lower! My comment is:
Re: [The Publisher] WIND STRENGTH MYTH
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Psychologically a 20 knot wind in March, when it is COLD feels much more daunting that it does in balmy August temperatures, but that may be due to the contrast between an anticipated capsize into cold water lumbered with a dry suit and clothing, compared to a pleasant, if slow, swim in shorts in the summer.
At fixed pressure, the density of a gas is inversely proportional to the Kelvin temperature, i.e. Centigrade plus 273. The force on the sail is proportional to the rate of change of the momentum of the air, which in turn is proportional to the velocity squared times the density. For the same force on the sail, with the only change being the temperature of the air, the equivalent velocity is proportional to the square root of the Kelvin temperature. Then for a 5 degree centigrade change the equivalent to a 20 knot wind at 20 C (293 K) is a 20.17 knot wind at 25 C (298 K). It would take a change from 0 C (Iceboating) to 28 C to produce the quoted change from 20 knots to 21 knots (not 19 knots). The humidity also affects the air density and can change with temperature, so my conclusions strictly apply to dry air.
- Craig Leweck, Scuttlebutt