Wind temperature debate

First let me say that you have WAY too much time on your hands.
That being said, its an interesting debate. Air most certainly gets less dense as temperature rises. Aircraft are affected by this in the form of decreased lift and must use increased airspeed or shed weight to take off on a hot day. This is made even worse at higher elevations as air density decreases with altitude along with an increase in temp.
So, does this also affect the amount of "push" on a kite that any given wind speed has?
Does the sailing term "heavy air" refer to wind generated during denser air scenarios ie: colder or moisture laden?
Ananometers are not tied in or calibrated to temperature. Wind speed is simply that. A bubble floating along in 10 knot wind moves at 10 knots regardless of the temp.
What you are asking about is the amount of force generated by the moving mass of air.
Don't you love the way that I simply restated your question without contributing one iota to the answer? Brilliant!
Time for the science majors to chime in.

10 mph is 10 mph, regardless of the density. Dense air moving at 10 mph is the same as sparse air moving at 10 mph. The only way it would effect the kite differently is if it is very turbulent or gusty which would give you varying wind speeds, not constant 10 mph. Think of it this way, a vehicle traveling at 10 mph is going 10 mph, doesn't matter if it is a VW Bug or a semi tractor trailer.

Now the actual density of the air does effect the kite. Generally a kite will fly worse in cool or wet air than it will in dry air, the reason being is density. The thicker the air the more resistance the kite will have to move through it. Water replaces air so in a wetter climate, the kite will be more sluggish or hang back more than it would in a very dry climate just because the kite will have a harder time flying through the air because it is denser. Think of walking through clean water compared to walking through thick mud.

With power kites, max power comes from apparent wind, apparent wind comes from kite speed. The faster the kite will fly the more power the kite is capable of producing from the apparent wind. Denser air causes more restriction/resistance on the kite while it flies which could reduce apparent wind (max speed) and overall power. Dry air creates less resistance to the kite so it can reach maximum velocity and maximum apparent wind.

Denser air may give you more bottom end grunt because the kite will be dragging back in the power zone longer than normal, but overall power production would be lessened. Dry air would allow the kite to reach maximum speed and maximum apparent wind which should be maximum overall power. This is one of the reasons that land speed records are easier to achieve in the desert dry lake beds instead of other areas. Of course, skill has more to do with it than air density.

All of this is on the minimal scale and unless you were able to fly in both dense and dry air with the same wind conditions back to back, it would be very difficult to tell the difference.

Just my 2 cents anyways.....

Excellent map there thanks! A few spots in the J-land I didn't know about ... and I get to add one too

Quote:

Originally posted by william_rx7 I think I see what you mean, and I agree. IMHO, without any science, or too much thinking, the same wind at 30F feels 'stronger' than at 110F. Then again, there are too many other variables and time to compare snow-kiting winds to kitesurfing wind. On the landboard, to me, the same wind, in the same field on the same surface, feels stronger in colder temps, but only a very little bit. (like less than 1/2 inch on the trim strap of my 12m or 7m Frenzy). I could be totally wrong, about this, and am probably brain washed by 'common knowledge'...

lighter moist air

Wind meter

The simple answer is that, no, temperature will not affect an anemometer, but yes, it will affect a kite. The simple explanation for why that happens is because the anemometer measures wind speed, which doesn't change with temperature, while the kite uses energy from a moving mass of air, which does change with temperature. That's because temperature only affects density, and density only affects the kite. If simple was all you wanted, stop reading.

The actual reasoning gets a little more complicated, but it's not that bad. Air temperature won't affect the anemometer because it just measures wind speed, which is independent of temperature. The fan blades will match the speed of whatever air mass is moving past them regardless of density. True, as some of you guys have pointed out, a given volume of air will impart less power to each blade, but once the blades have gotten up to speed it's not going to matter. There's so little resistance on the blades that they'll match the wind speed regardless of density.

mgatc mentioned a soap bubble floating in the wind, which will float at the same speed regardless of air density for the same reasons. If it's traveling slower than the wind, air molecules behind it will knock into it and speed it up. If it's traveling faster than the wind, it will knock into air molecules ahead of it and slow down. If it travels at the same speed as the wind, no force will be exerted on it, and, as Newton's Laws tell us, an object in motion tends to remain in motion. The anemometer's fan blades work the same way; it just measures wind speed.

The kite is a different story. Unlike the anemometer, it's anchored and providing a significant amount of resistance. The amount of air that hits the kite is definitely going to affect it, and a lower temperature means that there's more air hitting the kite at a given wind speed than there would be at a higher temperature. More air means more mass and more mass means more push. Keep in mind that the volume of air caught by the kite doesn't change here, but only the density of that air. Higher density means more air molecules in a given area, and that's why there's more air hitting the kite in a lower temperature.

awindofchange posted that a VW Bug traveling at 10mph is the same as a semi-truck traveling 10mph, but that's only to the radar gun measuring them (analogous to the anemometer). When they hit a barrier at that speed (analogous to the kite), you can bet there's going to be a significant difference. Although the speed was the same, the semi-truck packs a ton more power. Heavier air will impart more power to the kite than lighter air - true, that added density will increase drag, like awindofchange says, but it will also increase lift. For any kite with higher lift than drag (read: any kite that flies), added density will give you more lift and more power. Boats move faster in cold water because their propellers push an extra amount of water that more than compensates for the increased resistance in front of them, and planes fly better because they generate an extra amount of lift that more than compensates for the added drag.

How do we know all this happens? Physics - namely, the Ideal Gas Law. The Ideal Gas Law states that pV=nRT, where p is the pressure of the gas, V is the volume of the gas, n is the amount of gas (i.e., number of molecules), R is the gas constant (for this, it's enough to just know that this number doesn't change), and T is temperature. The variables that won't change here are n and p. n doesn't change because we're not adding or removing air molecules, and p doesn't change because air pressure will remain more or less constant for a given altitude (the wind only blows to equalize pressure).

So what can change? T and V. When T rises, V rises to balance the equation. This spreads the same n over a larger V. In plain english, when the temperature rises, it spreads the same number of air molecules over a wider area. This means that a kite of a given sail size will catch more air molecules in a given period of time, meaning that the kite will generate more power.

Colder air is more dense, so a given volume of it will provide more power than the same volume of warmer air, which affects the kite but not the anemometer. Think about sticking both underwater in a river - same fluid dynamics, very different density. The anemometer will still measure the speed at which the water's moving, but the kite is going to catch all that water and pull way harder than it would in wind moving at the same speed. That's why acampbell's example plane takes off in a shorter distance during colder weather even though the anemometer reads the same, and why bourck needs a different anemometer reading at different altitudes in order to launch the same kite.

Here's hoping that all made sense...