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When you get wind speed from an ASOS/AWOS or from ATC, are those speeds in true airspeed? Are the instruments weighted for elevation?

Say you're on approach to 05R at MEX and you're given wind speed of 25 kt at 140° and the OAT is 35°C. Do you need to take density altitude into account to determine if it's safe to land? Or is there not enough of a difference to matter?

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  • $\begingroup$ The "wind speed" is not an instantaneous value. It's the mean value of 24 measures which are themselves a mean value over 5 seconds (total is 2 minutes). The measures are done at 10 m AGL, but if they were done at 5 m, the values would be 13% smaller due to viscosity. Also the wind vertical component is not taken into account. This gives an idea of the accuracy, still I wonder how are anemometers calibrated. Good question. $\endgroup$ – mins Nov 4 '17 at 13:59
  • $\begingroup$ I think some of you confusion stems from learning about calibrated, indicated, and true airspeed of an airplane. Indicated and true airspeed vary with elevation because the number of air molecules hitting the pitot tube at high altitudes is less than at lower altitudes. That’s why there is a correction factor for altitude. As explained in @ymb1's answer you don’t need to do that when measuring wind speed. $\endgroup$ – JScarry Nov 4 '17 at 15:02
  • $\begingroup$ @JScarry: Still Tom is right when he says the effects of the wind depend on air density (contrary to what is said in the answer). However for a cup anemometer, air acts both on the concave retracting cup than on the two other advancing cups (which show their profile or their convex side) and I suspect the opposed forces cancel out most of the density variation effects. I'm not sure this would be the case on a propeller-type anemometer. $\endgroup$ – mins Nov 4 '17 at 15:22
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Wind is measured in true airspeed. That's why whether it's surface wind or a jet stream, subtracting the head/tail wind component from the airplane's TAS yields the ground speed (tailwind being a negative value). And a 25 knots crosswind—the example you give—will create the same crab angle for the same TAS being flown. But since pilots fly using IAS, the crab angle will be smaller the higher the airport.

Cup

Think of a cup anemometer, if the air is lighter (high altitude, high temp), there will be lower resistance and the same wind speed will result in the same rotation:

The remarkable thing is that the calibration does not depend on the air density. Who would have guessed? The cup anemometer responds to wind forces, and those wind forces have a magnitude directly proportional to density. Yet because in a steady state those forces are balanced, density vanishes as a factor.

The same thing applies to the propeller-vane anemometer.

Tube

In tube anemometers (pitot-static on a stick), corrections are applied:

[Due] to differing temperature, elevation or barometric pressure, a correction is required to obtain the actual wind speed. Approximately 1.5% (1.6% above 6,000 feet) should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level.

Ultrasonic

Ultrasonic anemometers are like cup anemometers, density is cancelled out since measurements are taken in multiple directions:

[Scalar] values (temperature, humidity, density) are eliminated and the result comprises wind speed.

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Surface winds are given in magnetic headings.

In contrast, winds aloft forecasts are given in true headings. They are not adjusted for DENALT.

Indicated airspeed (IAS) is the measurement, subject to instrument error. Indicated airspeed is really just a dynamic pressure reading.

Calibrated airspeed (CAS) compensates for instrument errors.

True airspeed (TAS) is CAS adjusted for air density (pressure and temperature). TAS does not take into account winds, and therefore is not a true ground speed.

Surface winds are given in a ground relative flow of air, and are relative to a magnetic heading.

Winds aloft are forecast and reported in an airspeed, and are intrinsically based upon the ground relative flow of air, and are relative to a true heading.

Therefore the AWOS/ASOS/tower reported surface winds are not in TAS, as that term is defined as being a vehicle vector magnitude.

However, when at Leadville, CO, (KLXV) the measurements of the airflow are relative to a datum slightly above the ground, and in that sense are analogous to the true airspeed of an aircraft if it were flying stationary above the airport. But TAS is not the correct term. It is simply a surface wind velocity and the corresponding magnetic directional heading.

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  • $\begingroup$ I didn’t figure that TAS was the correct term, but I couldn’t think of any way to express it. But you get the point, is it analogous to TAS or is it affected by altitude. I used a conversion calculator on the web and found that at KLXV elevation, 30kt TAS converts to 25.8kt EAS, so a 14% difference. Not sure how much difference that would make in the scheme of things. It will effect the flight characteristics of the plane by 25.8kt instead of 30. A hot day might make even more of a difference. $\endgroup$ – TomMcW Nov 6 '17 at 23:43
  • $\begingroup$ The crosswind of 20k at KLXV will have the same effect on your plane as a crosswind of 20k at KMIA. $\endgroup$ – mongo Nov 7 '17 at 0:36
  • $\begingroup$ What about head and tailwind? I'm thinking, if you're sitting on the runway at sea level facing a 30kt headwind your IAS shoulda read 30kt. But if you're at KLXV that same headwind would read 25.8. So you'd need a slightly longer takeoff roll to reach Vr. Or am I thinking wrong? $\endgroup$ – TomMcW Nov 7 '17 at 17:31
  • $\begingroup$ 30 kt at KLXV in @TomMcW question $\endgroup$ – mongo Nov 7 '17 at 18:23

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