Wind Speed and Density Theories

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Postby berrisbob » Sat Oct 13, 2007 11:34 pm

but warmer air can carry more water vapor (is more humid), thus making it "thicker" . . .


Actually, the density of air goes down as the amount of water vapor increases. The reason -- as water (H20) vapor increases, Nitrogen (N2) and Oxygen O2) decreases.

Given the weights of the constituent atoms:

H = 1
N = 14
O = 16

Then the corresponding molecular weights are:

H20 = 2 x 1 + 1 x 16 = 18
N2 = 2 x 14 = 28
O2 = 2 x 16 = 32

So as the lighter H20 ( 18 ) replaces the heavier N2 ( 28 ) and O2 ( 32 ), the less dense the air becomes.

Generally, the inland air in the Bay Area is relatively dry (higher density), but also relatively warm (lower density). Conversely, the coastal air is relatively humid (lower density), but cooler (higher density). So the temperature and humidity conditions in each location tend to cancel each other out. From this simple qualitative analysis, it's impossible to determine which location then would typically have the denser, more powerful, air. For that, someone needs to actually crunch the numbers. Consumer, this is right up your alley. Are you still out there?
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Postby davewsail » Sun Oct 14, 2007 7:57 am

Holy shit Scott, you must be really bored! This is what happens when the wind shuts down. Sure you don't want to go to Costa Rica?
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Postby OliverG » Sun Oct 14, 2007 9:35 am

All I know I that during the regular season, 20 on the coast seems more powerful than 20 at the delta...
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Postby timwim1 » Sun Oct 14, 2007 11:31 am

Assuming this was measured with the same windmeter... :). This would be excellent starting point to start the theoretical background....to explain why. Temperature is probably the key...once I have some time, I will make some calculations, with some typical situations. Tim
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Postby berrisbob » Sun Oct 14, 2007 12:49 pm

Since this post has now become completely hijacked, and since, as Dave has surmised, I am completely bored...

Tim brings up an interesting point - "Assuming this was measured with the same windmeter..."

In reality, a wind meter doesn't measure wind speed at all. It measures the force, or pressure of the wind. The force of the wind is proportional to the the air density and actual speed, which is unknown. In fact, if you took a wind meter that was properly calibrated for standard conditions on Earth, and used it to measure the wind speed on Mars, you might find that it also reads 20mph, when the actual wind speed might be several hundred miles per hour (not that uncommon for Mars). This is because the atmosphere is so much thinner on Mars.

The important thing about this thought experiment is that a land sailor on Mars with a 200mph wind measuring 20mph on a given wind meter would feel similarly powered to a land sailor on the playa at Burning Man when the same wind meter was indicating 20mph with an actual wind speed of essentially 20mph. The bottom line is that the important factor is the force of the wind, not the actual wind speed.

To Ollie's point: "All I know I that during the regular season, 20 on the coast seems more powerful than 20 at the delta...". If it were the same wind meter reading 20mph, and in the actual location of the riding (i.e. on the water in the riding zone and not somewhere on land), then a rider should feel equally powered, even if the actual wind speeds, due to differences in temperature and humidity, are a couple of mph off from 20mph, even in opposite directions. I've heard a lot of people echo Ollie's observation. And I've noticed myself that I can feel overpowered at Waddell on a 10m kite when the wind graph shows a steady 20mph, yet at the delta I'm more likely to be comfortably powered on a 12m when the graph says 20mph. But I believe that is more likely attributable to differences in calibration between the 2 sensors, or their locations relative to their respective riding zones, and not to the temperature and humidity characteristics of the air at their sites.

OK, I'm going to go for a bike ride now in this beautiful weather, and try to convince myself that it is as much fun as kiting. :|

Oh, and yeah Dave, I do want to go to Costa Rica. I just can't. :cry:
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Postby ramsey » Sun Oct 14, 2007 1:03 pm

Hey berrisbob, not to burst your bubble or anything, but even though the atomic weight of water is less than N2, water vapor is about 1000 times more dense than air that means mass per volume. So humid air is denser, colder humid air is even more dense.

I'm bored too can we get some wind please...........
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Postby davewsail » Sun Oct 14, 2007 1:44 pm

OK, now you've all dragged me into this thought experiment in my own boredom. Actually Scott is right about humid air being lower density than dry air. From wikipedia...

Image

Where:
Image Density of the humid air Image

pd = Partial pressure of dry air (Pa)
Rd = Specific gas constant for dry air, 287.05 Image

T = Temperature (K)

pv = Vapor pressure of water (Pa)
Rv = Specific gas constant for water vapor, 461.495 Image

The total pressure equals the partial pressure of dry air plus the partial pressure of water vapor. Therefore because Rv (water vapor) is larger than Rd (dry air), the smaller the overall density becomes as the partial of water vapor increases. So Scott's simpler analysis based on molecular weights does indeed hold.

If anyone is really bored, wikipedia (look up "air density") provides all the equations you need to calculate what the different air densities would be for various temperatures and relative humidity levels.

This whole thread reminds me of a Seinfeld episode... George decides to cut sex out of his life and all of a sudden he becomes a genius, super productive, and successful. But the moment he has sex again, he turns into the same old bumbling idiot we all love. Now just substitute "wind" for "sex" and one of us on this thread for George and walla-- now you know why this thread has become what it has. Deep thought... what would our lives be like if there was no kiteboarding?
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Postby adamrod » Sun Oct 14, 2007 2:39 pm

anybody know a kid in high school who wants to win the science fair? this has the makings of a great science fair project. attach a fan to an electric motor, measure the current generated when you blow air over it, and then vary the temperature and relative humidity of the air. Most likely Dave is right, but it'd be a great little project. specifically, even if Dave is right, we don't have an idea of the magnitude of these effects . . .
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Postby dakine1 » Sun Oct 14, 2007 4:58 pm

WOW.. you guys are scaring us inland kiters..... Way to much info.... Lets kite....Anybody want to go squirrel hunting???? :shock:

ED/OUT
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Postby OliverG » Sun Oct 14, 2007 5:35 pm

Just make sure you have your costumes ready for Saturday... :!: :drinkers: :!: :snakeman: :!: :supz: :butthead:
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