Overspeeding is a positive bias on the measured mean wind speed \(U\) caused mainly by two properties, the atmospheric turbulence and the asymmetric construction of the rotor. The asymmetry exists simply because the wind forces are larger with the wind blowing into the cups than with the wind blowing on their backsides. This is essential for the working of the cup anemometer because without this difference the anemometer rotor would never start rotating. The reason that the turbulence plays an important role is that the anemometer is calibrated in a wind tunnel with no, or very little, turbulence. So when exposed to turbulence in the atmosphere the cup anemometer will react to the wind fluctuations from a whole spectrum of eddy sizes. An anemometer with a small distance constant \(\ell_\circ\) can easily follow the larger eddies, but when eddies are of sizes equal to or smaller than the distance constant the anemometer cannot follow the fluctuations. However, these fluctuations will cause the rotor to spend more time on the plus side than on the minus side of the mean wind speed \(U\), compared to the calibration condition in the wind tunnel with no turbulence. A special, but quite common, case is the signal from an anemometer in a windy surface layer. The overspeeding relative to the mean-wind speed is \(C(\sigma^2/U^2)(\ell_\circ/z)^{2/3}\). Here \(\sigma^2\) is the variance of the wind speed or the square of the standard deviation, \(z\) the height and \(C\) a dimensionless constant of the order of one.

It is possible to use the cup anemometer to measure the variance and higher-order moments. But these moments will show no bias.