Cups, Props and Vanes

We discuss the dynamics of the cup anemometer, the propeller anemometer and the wind vane. The phenomenological model by Kristensen (1993) is modified to describe the motion of the propeller anemometer. We use the wellknow second-order differential equation describing the motion of the wind vane to study the properties of the vane itself and the vane in combination with a cup and a vane. It is argued that even the simplest question about how to record the signal from a cup anemometer has an ambiguous answer and can lead to a, not necessarily small, systematic error in the measured mean wind speed. We show how it is possible to measure the entire wind direction variance by utilizing the fact that an underdamped vane by ‘overshooting’ may compensate for the highfrequency variance loss if the damping coefficient is about 0.4. Finally we discuss possible sources of bias on the measured mean-wind speed when using a propellervane anemometer. It turns out that this anemometer has the same type biases from the lateral and vertical wind fluctuations as has the cup anemometer. In addition there are bias contributions from misalignment between the mean wind and the propeller axis and from the translatory motion of the propeller itself with respect to the vertical wind-vane axis. ISBN 87–550–2006–2 ISSN 0106–2840 Information Service Department · Riso · 1994