To gain an accurate indication of the generating capacity of turbines installed on a particular site it is necessary to monitor the wind resource. As wind speed increases, and becomes less turbulent with height the monitoring mast needs to be a similar height to the proposed turbines. The more data that can be collected the better the dataset so a period of 6 months to 1 year is usually recommended as a minimum. The data set can be extended by correlation with 20 year data from a nearby mast operated by the meteorological office.
Data is collected at more than one height above the ground to allow assessment of wind shear and turbulence, and duplicate instruments are often used to guard against failure.
Whilst monitoring is obviously desirable it is often not done due to the following factors:
1. Confidence that there is adequate wind resource
2. Time delay, possibly reducing chance of success in the planning system
3. Cost of installing a mast, and subsequent data collection
Notwithstanding the above a full wind resource report will allow the developer, and the source of finance, the ability to better assess the risk of investment in turbines at a specific location.