## 3 Surface-Based In-Situ Measurement Techniques
### Rotating Anemometers  
Rotating anemometers comprise cup and propeller anemometers (Figure 2). The latter is typically a combination of a propeller and a wind vane, providing both the wind direction and speed. Furthermore, cup anemometers have been introduced as combined systems with a wind vane [19], although more often, they are deployed as separate instruments.  
The cup anemometer is an old but thoroughly tested technology that, even more than 150 years after its design by [PERSON], is being improved via new research [58]. Cup anemometers have many advantages compared to other instruments. They are robust; the measurement accuracy is high; they are easy to apply, relatively inexpensive and can be run over long periods without maintenance and/or  
Figure 2: Examples of rotating anemometers, from the left: (**a**) propeller anemometer Wind Monitor Model 05103 by R. M. Young Company; (**b**) cup anemometer P2546-OPR by WindSensor; (**c**) combined cup-vane anemometer Wind Sensor WM30 by Vaisala; and (**d**) cup anemometer and wind vane WA25 Wind Set by Vaisala.  
-recalibration. Cup anemometer calibration is a well-known procedure [22; 23]. The measurement principle is simple: the angular velocity (\(\omega\)) of the anemometer is a linear function of wind speed over most of the wind speed range (except close to the starting speed, which is nowadays typically less than 1 ms\({}^{-1}\), i.e., practically unimportant for wind gusts) [19]. The anemometer factor:  
\[f_{a}=\frac{u}{r\omega}, \tag{16}\]  
where \(r\) is the cup anemometer rotor radius (distance from the axis of rotation to the center of a cup) and \(\omega\) the rotor rotation rate. \(f_{a}\) depends on the design of the anemometer and varies typically from 2.5-4.5 between different anemometer types (e.g., [19; 21]).  
Although the cup anemometer calibration is simple and linear, its dynamic response to wind fluctuations is highly non-linear (e.g., [18; 19; 20; 21; 59]), i.e., the anemometer responds faster to a wind speed increase than to a decrease. This effect is mainly caused by the lateral wind velocity component (horizontal wind component perpendicular to the mean wind direction) [19], and it affects mostly the mean wind speed when averages are taken over long periods: the asymmetry of the response causes a positive bias, called overspeeding, in the resulting mean wind speed. However, we depict that overspeeding is less important for wind gusts than for the mean wind speed, as the wind gusts represent an average over a few seconds only. Instead, the more important parameter for wind gusts is probably the response length \(l\) of the cup anemometer (introduced in Equation (14)). It is the length of the air volume that has to blow through the anemometer for this to reach \(1-e^{-1}\approx 63\%\) of its final reaction to the change in wind speed. Like anemometer factor \(f_{a}\), it depends on the anemometer design, but also on the density of the rotor material in relation to air density (\(\rho/\rho_{a}\)): rotors made of heavy material react slower than light-material anemometers. Moreover, as \(l\) is inversely proportional to air density (\(\rho_{a}\)), the instrument may react 10-20% slower at higher altitudes compared to that at the sea level [19]. Cup anemometers used at weather stations (Figure 2) have a response length \(l\) of about 2 m, which means that at wind speeds of 10 ms\({}^{-1}\), the cup anemometer measurements are sensitive at time scales down to 0.2 s, i.e., the measurements are representative of wind gusts when the gust is calculated as an average over 3 s. At very low wind speeds, the anemometer response may affect short averages of wind speed, but on the other hand, the main aim of measuring wind gusts is not for weak winds.  
The dynamics of a propeller anemometer resemble that of a cup anemometer with a linear calibration and a non-linear response to wind speed fluctuations. However, the overspeeding is less serious for propeller anemometers than for cup anemometers [18].