Wind speed

[Swampfox201]

Question about wind speed.

Wind speed is measured by using an anemometer which is acted upon by the air molecules striking the device and making it turn. The faster it turns the faster the wind.


Lets say we are at sea level and the wind is blowing 10 kts. Lets also say then that each air molecule is moving across the ground at 10 kts.

Now lets go up higher. Lets say we are at an airport where the air is less dense than the sea level airport. I'm not sure how high it would have to be but lets say the air here is 1/2 as dense as the sea level airport. Again lets say the anemometer is showing 10 kts.

My question is if the air is half as dense then even though the anemometer shows 10 kts are the individual air molecules actually moving 20 kts across the ground in order to apply the same force to the anemometer?

For you out there that say "Who gives a rat's a**!! The airplane doesn't care" you would be right. Just a question.

Todd

 

[Captain]

Ummmm...wind speed isn't given as a TAS bot as an IAS. Ummm...isn't wind speed given as an IAS and not a TAS? That way the number of air molecules going accross the anemometer equal the number going over your wing minus aircraft speed?

I've never thought about that before but that's my guess. The anemometer turns at X rpm at 10 knots of wind TAS at sea level. At altitude that would mean X minus a tad but is offset by the fact that you're flying a faster TAS because your airspeed indicator indicates indicated airspeed so that makes it a wash.

There's my guess. I could be wrong. I would, however, like to accept my award for using the same word three times in a row and having it still make sense. Where do I pick that up?


screwed up first sentence. It was supposed to be a question.

 

[N801BH]

That is a good question you posted.... The easiest way to answer is... yes,,, the higher you go the less dense the air is...That is why there are TAS = true air speed that corrects for density and IAS= indicated air speed.. it is ALWAYS lower then true airspeed... I have Davis weather station here at home and when I ordered it I specfically asked to make sure it was caliberated correctly... When it got here I made a mount that bolted onto my PU truck and suspended the anamometer 10 feet up away from the cab so it didn't get influence from the airflow off the truck and read "clean" air.
I picked a no wind day and drove back and forth for 2 miles on a road, using my Garmin GPS to display ground speed in case the speedometer in the truck was inaccurate.. Took all the numbers and averaged them out... it showed 13% low... I am at 6450 MSL.. Davis Instruments is at 55 MSL. it was easy to correct as that weather station has a cal number setting to correct for thin air.... YMMV.

 

[olasek]

My question is if the air is half as dense then even though the anemometer shows 10 kts are the individual air molecules actually moving 20 kts across the ground in order to apply the same force to the anemometer?

No.
It is not 20 kts, it is 14 kts.
This is because the dynamic pressure is proportional to the air density and to the velocity squared. A bit of elementary mathematics and using square root of 2 which is roughly 1.4 makes it 14 kts approx.

 

[pericynthion]

Off the top of my head:
* Cup-type anemometers measure dynamic pressure, proportional to density * velocity^2
* Hot-wire anemometers measure air mass flow, proportional to density * velocity
* Ultrasound anemometers measure true air velocity, possibly with a temperature correction needed due to its effect on the speed of sound.

As for what's reported by an automated station or in a METAR, I would guess that it is the true velocity, but I'm not sure and would like to see a reference. Since conversions are easy to do, or to build in to the system's calibration, I don't think that looking at the mechanics of the system is going to tell you how whether true or indicated wind speed is reported.

 

[Swampfox201]

Thanks for the replies. I've been thinking about this some today.

I understand about TAS vs IAS and how that changes with density altitude. Most pilots here understand that and how it relates to flight.

Lets forget flying and keep ourself on the ground. What does the wind "feel" like to ourselves and our surroundings? At a higher elevation the air must move faster to have the same effect as at a lower elevation. For instance at sea level my trash can will blow over at 20 kts but in Denver the air must be moving faster than 20 kts to do the same thing.

I don't know squat about balloons but lets say I'm a balloon pilot and I'm used to flying out of high elevation fields. I know that my limit to inflate is 15 kts. Any more and I will damage my balloon. I use Ben's "calibrated" anemometer from his post above to determine the winds. Now lets say I go to fly at sea level. The anemometer there says 15kts so i inflate my balloon and it gets damaged because the wind force at sea level 15 kts is greater than Ben's calibrated 15kts.

My question is why do we calibrate these things? Wind should be measured by force and not by how fast its actually moving.

 

[Piloto]

Essentialy there are two type of anenometers. The ones that measure dynamic air pressure like those in airplanes and those that measure air velocity independent of density like ultrasonic anenometer. For air planes dynamic air pressure is more valuable than air velocity because it is a direct function of the potential lift force. While air velocity is not derived from air pressure but from the actual velocity of the air molecule. Ultrasonic anenometer measure the actual sound propagation velocity through moving air.

José

 

[pericynthion]

My question is why do we calibrate these things? Wind should be measured by force and not by how fast its actually moving.

Depends. For navigation and fuel planning you definitely care about the true wind velocity. Landing performance kinda needs "both".

 

[jasc15]

No.
It is not 20 kts, it is 14 kts.
This is because the dynamic pressure is proportional to the air density and to the velocity squared. A bit of elementary mathematics and using square root of 2 which is roughly 1.4 makes it 14 kts approx.

Dynamic pressure is indeed proportional to the square of speed, but is proportional to density. So, in the OP's question only the density is changing not the speed, so the dynamic pressure would be halved, all else being equal. If these anemometers measure dynamic pressure, it would read half the sea-level value.

 

[gismo]

Thanks for the replies. I've been thinking about this some today.

I understand about TAS vs IAS and how that changes with density altitude. Most pilots here understand that and how it relates to flight.

Lets forget flying and keep ourself on the ground. What does the wind "feel" like to ourselves and our surroundings? At a higher elevation the air must move faster to have the same effect as at a lower elevation. For instance at sea level my trash can will blow over at 20 kts but in Denver the air must be moving faster than 20 kts to do the same thing.

I don't know squat about balloons but lets say I'm a balloon pilot and I'm used to flying out of high elevation fields. I know that my limit to inflate is 15 kts. Any more and I will damage my balloon. I use Ben's "calibrated" anemometer from his post above to determine the winds. Now lets say I go to fly at sea level. The anemometer there says 15kts so i inflate my balloon and it gets damaged because the wind force at sea level 15 kts is greater than Ben's calibrated 15kts.

My question is why do we calibrate these things? Wind should be measured by force and not by how fast its actually moving.

Sounds to me that what you "know" is that 15 Kts of wind is your limit at your high elevation. And since that's likely a force (dynamic pressure) issue you would be wise to adjust your limits for a sea level airport. OTOH the adjustment from 8000 MSL to sea level would be a whopping 2 kt, something that's probably less than the inaccuracy of your local wind speed measurement.

IMO life would be simpler if whoever invented the "airspeed" indicator had marked it's scale with dynamic pressure (square root of the dynamic pressure would be even better) given that's what it really shows. Outside of the rarely seen in flight sea level DA the ASI never shows a "speed" of any sort.

 

[pericynthion]

I guess IAS always equals TAS for Henning, 50 ft above the waves

 

[Sac Arrow]

Both cup type and ultrasonic anemometers give "direct" speed measurements. Pitot devices like your airplane's IAS indicator are subject to density variation.

Caviot - a cup type anemometer does have mass and bearing drag, so it is somewhat effected by air density, some more so than others but not proportionately like a pitot gauge.

 

[pericynthion]

Both cup type and ultrasonic anemometers give "direct" speed measurements. Pitot devices like your airplane's IAS indicator are subject to density variation.

Caviot - a cup type anemometer does have mass and bearing drag, so it is somewhat effected by air density, some more so than others but not proportionately like a pitot gauge.

Nice - learned something. Gory details here.

 

[Swampfox201]

Sounds to me that what you "know" is that 15 Kts of wind is your limit at your high elevation. And since that's likely a force (dynamic pressure) issue you would be wise to adjust your limits for a sea level airport. OTOH the adjustment from 8000 MSL to sea level would be a whopping 2 kt, something that's probably less than the inaccuracy of your local wind speed measurement.

What got me thinking was a article about wind storms on Mars. The article mentioned the wind speed and then said the Martian atmosphere was something like 1% the density of Earth's. So is the Martian wind actually moving x mph or is it producing the same force as an x mph earth wind???