It is common practice to compute wind run and mean wind by counting the total number of anemometer rotations in a measured time. The practice assumes that each turn of the rotor signifies the passage of a fixed run of wind, or that the anemometer's performance is expressed by S = bR, where S is wind speed in meters per second, b is a coefficient with units of meters per revolution, and R is the turning rate in revolutions per second. Tests in NCAR's wind tunnel show that, although true helicoid anemometers obey the equation S = bR, all other types of rotational anemometers obey the equation S = a + bR. The significance of errors due to this oversight is discussed.

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1 The National Center for Atmospheric Research is sponsored by the National Science Foundation.