Wind strength/temp

[Ex-Calif]

Therapy has the theory. The sail is an airfoil. Lift is generated by the mass flowing across the convex surface. More mass at same velocity = more lift.

Aircraft have this exact problem. Due to the lower density, hot, high altitude airports require more speed and hence more runway to generate enough lift to produce flight. Interestingly, the less dense air also has less effect on the speed indicator so the indicated airspeed is roughly the same. True airspeed, corrected for temperature and altitude is higher.

There are lot's of complexities but Bernoulli & Newton pretty much figured it out.

At the molecular level, denser air has more molecules acting on the sail.

[MarkJ]

Dan,

Do you think we could actually notice the difference? Say between 20 kts wind at 35 degrees C and 20 kts wind at 10 degrees C?

mark

[Ex-Calif]

Indicated windspeed using standard pressure instruments is already adjusted for density so, 20kts is 20kts. The boat knows no different.

[Moonchaser2304]

Does 20% humidy feel drier at 10,000 foot elevation than 20% humidity at 3,000 foot elevation at the same temperature?

[Ex-Calif]

If a wind blows in a forest and there is no sailboat around, does it make a noise?

When I am at 10,000 feet I am usually flying with the heater on...

What we feel is interesting but probably irrelevant. Humidity, temperature, wind pressure and speed are all measurable characteristics of the soup we live in. What we feel is probably clouded by our acclimation and past experience.

People arrive in Singapore and complain of heat and humidity. I go back to the california in the fall and put on a jacket when the temperature drops below 70F.

[Alan Wheeler]

The result was a tide of over six feet above high,

Arrr now "Therapy". Thats different again. Storm surges come from Air pressure. Air pressure is a different beast compleatly. Air pressure is the weight of air forcing down on the earths surface. During a storm, the Air pressure is often much lower and so the ocean actually rises. This is what creates the storm surge. Yes wind can drive water along too and the combination can be catastrophic.
Interestingly, cold fronts are usually associated with low pressures area's. High pressure area's are usually associated with fine weather. The reason the air pressure is high, is that it is cold air falling to earth from extreme altitude.

20kts is 20kts. The boat knows no different.

Yes true Dan. But we are not talking wind speed. We are talking about mass. To take it to extreme for an example. It is like a cubic metre of air hitting you at 20Kts and a Cubic metre of water hitting you at 20kts. Both are the same speed. But each a very different mass and the water is going to knock you off your feet.
I forgot about the Aircraft thing. I remember now, an airline disarster many years ago (maybe even before my time), in India or somewhere I think. It crashed due to the temp being so high, that it simply didn't get off the deck before it ran out of runway.

[roverhi]

Being an old Navy Jet Pilot, had lots of experience with air density. Our take off roll at 30 degrees was half what it was at 90 degrees. That was a combination of greater engine thrust because of more Oxygen per cubic foot of air allowing more fuel to be burned per rpm and the denser air creating more lift.

Now whether it would be enough of an effect to really notice in a sail boat is problematic. Do know that wind and waves are way stronger and bigger when it's dark and/or overcast. Since it's more often overcast and dark in the winter, would have to go with the denser air having way noticably more force. Have often thought how much more challenging it would be sailing the channels between islands here if it wasn't always sunny and warm.

Aloha
Peter O.

[Ex-Calif]

"But we are not talking wind speed. We are talking about mass. To take it to extreme for an example. It is like a cubic metre of air hitting you at 20Kts and a Cubic metre of water hitting you at 20kts"

Well you are really talking about volume.

Mass is the fundamental property. It's hard to relate mass to anything else. Commonly we will use density. Density = Mass / Volume

Most elements (solids) are very stable over temparature ranges. Most gasses are not and need to be corrected for standard temperature and pressure. Gases become denser (more mass per unit volume) with lower temperature. Water becomes less dense at lower temperatures.

Imagine the density of the water changing. Let's say we freeze it. The mass is the same, the volume is greater and I sure wouldn't want to get hit by a cubic meter of ice at 20 kts. (This has more to do with how much of the water mass is really hitting you vs. the ice mass and the conservation of energy rules.)

In your example if we kept the volume the same for the water, we would have to have less mass because 1 cubic feet of water won't fit in a one cubic foot box when frozen. Interestingly after we let the ice melt we have less potential energy in the water.

One cubic foot of dense (cold) air has more potential energy that one cubic foot of less dense (hot) air. There are two ways to use this density. Potential energy (at rest) and kinetic energy (moving). Kinetic energy of an air mass is most useful to us.

"Air speed" is a little misleading because we are using a "wind" instrument to make the measurement. By its nature the wind instrument depends on the density of the gas to operate. To make the anemometer read 20kts with less dense air we have to move more (volume) of it by the anemometer - i.e. the same mass - and it will read 20kts because regardless of density the anemometer is reading the kinetic energy in the air mass.

The kinetic energy equation is:

1/2mv^2

We already know that mass per unit volume changes with temperature. So if mass goes down and indicated v is the same what's happened? Well indicated v is not true v. We can watch a car drive by but we can't really watch a cubic meter of air pass by. What we have done to read 20kts is increased the Volume of air.

One of the main reasons we always wanted to fly higher is that the air is less dense and we could fly through it faster with less drag. it is not unusual to be indicating 200 kts on a wind pressure gauge and be flying at a true airspeed of 500 kts. True airspeed is an indication of the volume of the air flowing past the lifting surfaces.

[Alan Wheeler]

Well you are really talking about volume.

Volume is the amount of space something takes up.
Mass is the weight of something. Usually meaning, that something has more atoms in a given area.
A cubic metre of air and a Cubic metre of water are both the same volume. They take up the same area. But water has more mass because waters molicules are more tightly packed together. You can compress more air into the space, so you have more air molicles and more air molicles means more mass.
You may note that I have used both Atoms and molicules. They are different but the use of the term Mass encompasses both. In normal daily use, Mass is the expression of weight. But in scientific terms, Mass is actually a measurement of the gravitational pull of an object. As the Mass increase, so does the gravitational pull. Although science stil does not know what the force of gravity actually is, they do understand it to be linked to the Atom itself. ooops, I'm woffling.

[Ex-Calif]

As I have said before I am a translator of Engineering speak and by no means an expert myself but:

Mass is a fundamental characteristic of matter
Gravity is one of the four fundamental forces identified in nature
Weight = Mass X Gravity - this is why a solid object in space has the same mass but less weight

"A cubic metre of air and a Cubic metre of water are both the same volume. They take up the same area. But water has more mass because waters molicules are more tightly packed together. You can compress more air into the space, so you have more air molicles and more air molicles means more mass"

What you are describing is density.

density = mass / volume

The density of air is heavily influenced by pressure and temperature.

What we might be hung up on is the way we measure air speed versus solid objects.

Imagine your standard 4 paddle anemometer at the top of the mast. Move the boat across the earth at 20 knots on a (no wind) standard pressure and temperature day. A properly calibrated anemometer will read 20kts.

Move the boat to the edge of the atmosphere with the same velocity. The anemometer will read close to zero. The boat is still moving at 20 kts but there is insufficient mass in the air to propel the paddles. If you could count the air molecules there would be fewer molecules passing by.

The same thing happens on a hot day. The anemometer will read less than 20kts for the same volume of air.

However, when the anemometer reads 20 kts on that hot day you are effectively passing the same volume of air. This means the same energy is available and the same boat speed will result.

[Mr Blean]

Originally Posted by Therapy

Well what the forecasters and all forgot to take into account was the fact that the air temp was also dropping fast (and winds increasing as it came ashore. An unusual occurrence with that type of circulation at that time of year etc.

The result was a tide of over six feet above high, lots of homes/trailers flooded, some cold folks on roofs for a while, cars (and boats) sunk etc.

In the case of storm surges there is also the effect of air pressure. Believe it or not air is (relatively) heavy stuff (if you don't believe this, how come a balloon of less than a foot in diameter filled with Helium will float in air carrying not only the weight of it's envelope but also the string to which it is often attached?). So, when air pressure is low the atmosphere places less pressure on the ocean from above and the water expands upwards hence giving the effect of an abnormally high tide. Surges are also created when a large mass of water is blown from wide and/or deep water into a narrow and/or shallow area (such as an estuary). The result is a rise in water level. The worst flooding tends to occur when these effects are combined.

In terms of the original question not only is cold and more humid air more dense but I believe the equation for the motive force generated by air in an aerofoil may be to some complex function of air density (such as air density squared or even cubed) hence a small change in density equates to a much more significant change in motive force. I'm interested in this myself so will check it out and report back.

[GordMay]

The molecular weight of air is 29, whilst that of helium only 4 ; allowing the balloon to float in air.
Notwithstanding, nearly all matter (even most gases) is/are heavier than air, which is relatively light.
There are only 14 substances lighter than air, at the same pressure & temperature;
Neon
Hydrogen fluoride
Water (steam)
Carbon monoxide
Ammonia
Nitrogen
Hydrogen cyanide
Methane
Ethylene
Acetylene
Methllithium
Diborane
Helium
Hydrogen

[David M]

Originally Posted by rebel heart

So a cold football is supposed to have less density than a warm one? Sure as hell didn't feel that way. Maybe the leather was what made it feel like a brick, but cold footballs = painful.

I think you are right on with that one Rebel...it has more to do with physiology than physics. Nerves seem to be more sensitive to impact when they are cold. I would rather accidentally bang my hands against something when they are warm than when they are cold.

[MidLandOne]

We actually did all the calcs at one stage but not doing them all again - but turns out that as far as they were concerned for the sail boat situation the air temperature makes no difference worth considering at all as far as the resultant forces due to increased density are concerned (tho' maybe so if one is down to looking at the last 1/100th of a knot boat speed in America's Cup or something, but I wouldn't know about that ).

What confuses many is when the "weight of the wind" is referred to and then think that is due to the density difference between hot and cold air whereas it refers actually to the difference in wind speed and/or direction between the sea surface and the mast head ie the wind shear.

If there is significant wind shear so the wind speed is higher at the masthead the boat will appear to heel more for the wind strength felt on deck than it would if there was no wind shear. Wind shear tends to occur when the water temperature is colder than the air temperature and the physiology effects David mentions may be related to that, perhaps?

[brian and clare]

I was told by a bigtime racer guy here in Auckland that they used differnt sails for different wind speeds in different areas of the world, so a #2 jib in the tropics might not be the same as a #2 in a different part of the world. I kept waiting for everyone (whitbread sailors, etc. when the dumb American finally "bought" the joke. But not a joke apparently. After that there was some discussion of things like salinity of the water and other random stuff that affects racers but by then I was drunk.