<P> The new generation of sensors use sound waves to measure wind speed and direction . The measurement is based on the time it takes for an ultrasonic pulse to travel from one transducer to another, which varies depending on - among other factors - the wind speed . The transit time is measured in both directions for several (usually two or three) pairs of the transducer heads . Based on those results, the sensor computes wind speed and direction . Compared to mechanical sensors, the ultrasonic sensors offer several advantages such as no moving parts, advanced self - diagnostic capabilities and reduced maintenance requirements . </P> <P> NWS and FAA ASOS stations and most of new AWOS installations are currently equipped with ultrasonic wind sensors . </P> <P> Unlike all other measurements, which are made near between 3 and 9 feet (1 and 3 meters) above the ground, wind speed and direction are measured at 30 feet (10 meters). </P> <P> To determine visibility, automated airport weather stations use one of two sensor types: </P>

At what altitude are surface air temperature observations made with asos
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