MONITORING STATION INSTRUMENTATION AND MEASUREMENTS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 39

Figure 4-3. A sonic anemometer. Source: Campbell Scientific.
be traveled by a cylindrical volume of air passing through the anemometer for
the anemometer to record 63% of an instantaneous speed change. Anemometers
with relatively large distance constants may overestimate the mean wind speed in
turbulent conditions compared to other anemometers. This is because they tend
to respond more quickly to a rise than to a drop in speed. Sonic anemometers
are not susceptible to this overspeeding effect. Unheated anemometers commonly
used for resource assessment have distance constants ranging from 1.8 to 3.0 m,
while heated anemometers, which are bulkier because of their heating elements,
tend to have much larger distance constants.
• Response to Vertical Wind. In relatively steep terrain, the wind often has a
significant vertical component. Since wind turbines are sensitive only to the
horizontal component of the speed, and turbine power curves are defined and
measured under horizontal flow conditions, the vertical component should ideally
be ignored. Different anemometers have different characteristics in this respect.
The easiest to use are sonic and propeller anemometers. Mounted correctly,