Edited by Robert Repas
Anemometers simply measure wind speed. Many designs use the wind to spin a fan or other mechanical device. The speed at which the fan turns is then calibrated to the speed of the wind. In contrast, ultrasonic or, as they’re sometimes called, sonic anemometers measure wind speed using sound. Sound waves travel at a constant velocity through any medium. For air, that speed is approximately 1,100 fps. However, the velocity of any wind may add to or subtract from the velocity of sound. For example, if the wind blows in the direction of travel of a sound wave, the velocity of the wind adds to the sound’s velocity. Conversely, wind blowing against the direction of the sound reduces the sound’s velocity.
The design of an ultrasonic anemometer starts with two pairs of ultrasonic transceivers. One pair is aligned in a North-South direction (the NS pair), while the other pair is aligned in an East-West direction (the EW pair). Each pair measures the amount of time needed to send a sound pulse from one transceiver to the other in both directions. As the distance between transceivers is fixed, the velocity of the sound can be calculated from the elapsed time. In one direction the wind velocity adds to the sound velocity and subtracts from it in the opposite direction. Subtracting the velocity in one direction from the velocity in the other direction and dividing by two gives the velocity of the wind along each axis.
Summing the wind velocities for each axis determines the overall wind velocity. However, as the NS and EW axes are perpendicular to the other, one must use right-angle addition from the Pythagorean theorem: square each velocity, add them together, then take the square root of the sum.
Ultrasonic anemometers have an additional benefit not found in other types of anemometers. They can also display wind direction by calculating the arctangent value of the NS and EW velocities.