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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?
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.
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.
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 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.
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.
