I was having a conversation with someone this morning and he mentioned his feeling if the temperature was high, the windspeed didn't have the same "depth" as when it was cold.
i.e. 25 knots when the temp is say 34 deg does not feel as strong or dense as 25 knots when its only 18 deg.

Is there anything in that, or is it just that you feel more comfortable when the outside temp is warmer and dont get worn out so quickly?

Cold air is much denser than hot air i.e. a plane doesn't need to go as fast in cold air than hot to take off. I was once in Birdsville (2003) for work and we had to sit in the Pub for 3 hours until the airstrip surface temperature dropped below 50deg C so we could take off. We were there the day after they had a record 48deg in the shade( this was around the figure I recall)

Hence colder the air the denser it is which means a lot faster for equivalent wind speed. Someone please correct me as this is only an assumption.

am trying to digest sausages reply too but am sure there's more to it than just his assumption for example why do jet turbines work better in thin air that sort of thing

Looking at an extreme example. Take water and air. Air has a density of 1.2kg/m3, whereas water is 1000kg/m3. If you aim a stream of air at something vs a stream of water of the same velocity, the stream of water will have a lot more force.

Now so far as air density not effecting a wind meter, the wind meter is inherently designed to have as less friction as possible, hence the amount of force behind the air stream doesn't make any visible difference, as not much force is required to spin the little cups or fan in the windmeter. Windmeters measure wind velocity, not the 'power' of the wind.

Now as far as windsurfing goes, denser air means you can go faster, because in the same wind velocity there is more force/power in the wind, more force means you can overcome the drag forces of the water on your board and fins.

Just out of interest, air is about 7% denser at 15 degrees celcius, then it is at 35 degrees.

I can break out the proper maths if anyone cares :)

The gas equation PV = nRT explains it all. P is pressure, V is the volume the sample of air is squeezed into, n is the number of molecules of gas, R is a constant and T is the temperature relative to absolute zero - ie -273 dec C ( = 0 deg K). So if the pressure is the same and the temperature rises from say 10 degrees to 40 degrees that's 283 to 313 degrees K or a rise in temperature of 10%.

For the equation to hold the number of molecules in a cubic metre of air must drop by 10%, ie the density drops by 10%. That's 10% less momentum in air moving at the same speed - 10% less force generated so you can up your sail size by 10%.

A cup anemometer with good bearings is free wheeling so will measure pretty much the same windspeed regardless of temperature.

If you use a pitot tube you're measuring the pressure required to stop the air, you'll get a greater reading in cooler air, hence you need to apply a correction for density when using pitot tube based anemometers.

Air pressure also affects density and for a given temperature varies it by a few percent ie 990 hectopascals to 1020 hectpascals ~ 3 % variation there.

If you decide what sail to rig by looking at the whitecaps you don't have to worry about all this.

I think it's a matter of energy, (mass X speed), so the more energy in the wind, the more sail pressure.

Water is much more dense than air, so that's why the fin needs to be only a fraction of the size of the sail, to generate a similar force.

EDIT, Woops that's what happens when you get distracted, other people answer the question before you.
But I disagree with Swoosh, denser air doesn't necessarily mean you go faster, but it does mean you can use a smaller sail. The available force is never greater than the sailor can counterbalance.

I have read that the windmills get more energy from the colder denser air.
Personal experience has given me that feeling,although when the air flow is that cold it tends to work like a down draft and stick to ground level(water level),thus giving you the force where you need it.

Ian, I've been equally perplexed by that for many years, varying the square of windspeed, doesn't bare any resemblance to sail size changes.
And being only 70kg, I can't figure why I have to use almost the same size sail as my mates 20kg heavier.
Be nice if somebody can explain it.

Had a thought while doing the dishes. It's apparent wind that dictates the sail pressure. So reaching at 25 knots in 15 knots of wind with a 7.5 - apparent wind = 29 knots.

Reaching at 25 knots in 30 knots - apparent wind 39 knots so the theoretical sail size for
30 knots = 7.5 / ((39/29)^2 ) = 4.2 ~ about right.

Doesn't explain that other problem of big /little sailors in the same wind though Decrepit.

i'm onboard with the density thing now.

but it raises more questions.

density is related to temperature.

so shouldn't the density at 12 deg in victoria be the same as 12 deg in qld.

the other thing that no-one has mentioned is pressure gradients

Gestalt said...

i'm onboard with the density thing now.

but it raises more questions.

density is related to temperature.

so shouldn't the density at 12 deg in victoria be the same as 12 deg in qld.

the other thing that no-one has mentioned is pressure gradients

Pretty close, you'd just have to look at how much percentage change there is with atmospheric pressure at the two places on the day. There's also humidity which has a small effect. Remember the Aussie cyclist who claimed he lost out on a gold medal due to selecting a too high gear ratio. Claimed it was the humidity making the air denser. But a water molecule has an atomic weight of 18, nitrogen 28 and oxygen 32 - so more water in the mix at the same temperature and pressure makes it less dense. But I don't think it's by much.

sausage said...

I think a good example of this in layman's terms is when you put your hand out of the car window at 100km/hr (of course I never go over the speed limit). In hot weather you can hold your hand out easily, dipping it up and down (making aeroplane noises if you wish )........do the same in really cold weather and you have to turn the car around to retrieve your arm that was just ripped from its socket.

EDIT - I suppose you can deduce that this is why people in colder states are much slower than us Queenslanders, and this is qualified by the fact that they have daylight saving just to catch up with us.

Also this is why there are lot more one armed people from the south.

Another analogy. Take 2 equal sized balls, one made of styrene foam the other of lead. Drop them on your foot. Which one hurts more?
The denser air travelling at the same velocity "packs more punch"
Thats my take on it.

i think this thread proves a few things once and for all.

1. that 35 knot in qld is the same as 35 knots in WA. [}:)]

2. that qld sailors are faster due to the dissadvantage of less density in the wind

3. that north qld sailors are even faster again

4. that driving with your arm out the window is in fact an armless exercise.

5. that cold weather doesn't mean strong wind