Winds at 90F have more punch that the same winds at 70F?

[AlexM]

From today's iwindsurf daily briefing for the gorge: "Remember that W20-25 at 90F lacks the punch that these same winds deliver at 70F." Can anyone clarify what that statement means?

[ericandholly]

The difference is very marginal, but cooler air is more dense, thus has more mass per unit volume--ie, it "weighs more,"--so the same windspeed delivers more punch if the air is cooler. Again, not sure that there is enough of a difference for a sailor to ever notice.

[cgoudie1]

I can take a stab at it. 90 degree air less dense (locally) than
75 degree air. and sail force is determined (roughly) from
Bernoulli, so you've got less mass (of air molecules)
creating a lighter pressure differential to suck the sail along,
even though those molecules are moving at the
same speed. "Punch" of the initial gust front, causes your acceleration
to be greatly diminished with lower mass (and therefore air pressure).

What it really means is you need to rig about 1/2 meter bigger sail
in hotter weather, if you use an ultra low friction, or hot wire
anemometer to rig by. Me, I just use the wind I feel on my face and
the caps I see on the water to rig by, they're force driven like my sail.

-Craig

[Bond1]

[cgoudie1]

I had a look at the equation and Lift = Constant * (Density*
Velocity Squared/2) *Area.

Since the Area (of the sail) and Velocity (of the wind) and the Constant
are the same, the ratio of lift will be the same as the ratio of the
air density which for 90 and 70 degrees is about 4% in air at Sea level.
Can you feel a 4% decrease in power? Maybe, but I'm thinking the
variance in wind speed (in the Gorge) is probably more obvious, so
my initial suggestion of 1/2 meter bigger might be a bit large for
the 70-90 degree difference, maybe just tweak your outhaul a tiny bit.
;*)

-Craig

[moosesox]

It's a bit more subtle:

Density = Pressure divided by (Temperature and a constant). The constant is not relevant to this discussion.

The statement that wind at 90 degs packs less punch than at 70 degs needs to be qualified with "at the same pressure".

If the pressure goes up by the same amount, 90-deg wind can have the same punch as 70-deg wind. If the pressure goes up by more, then it can actually pack more punch.

Note that temperature, in this case, is absolute temperature. So, 90 degs is really 460+90 or 540 degs (Rankine). What this means for the math is that a 20-deg increase in temperature only requires roughly 4% increase in pressure to get to the same sail punch.

Science is fun, ain't it!!

Marc

[isobars]

Don't forget humidity. Greater humidity = less density = less power = larger sails if you're into second decimal points.

Then there's lunch, 600-grit sanding, PMS, and harness line length.

Mike \m/

[vikingsail]

Sorry but I can not resist,

so now the rigging gurus are going to add a barometer and thermometer to the spreadsheets, internet connections,laptops and windmeters to choose their sail size. And the first gust that hits will spoil their day.

Makes me wonder how Columbus ever made to the new world. ;->

Apologies to all, actually a good discussion of what pilots call density altitude.

[westender]

It means that when we are getting our asses kicked,, we blame it on the air density. If it's really hot, the danger is low. Your subject contradicts your message.