I've been reading with some interest the thread about smoothing the windspeed readings. For a while, I wrote software for a windpower company in the meteorology department. The wind must provide enough force to point the weather vane into the wind. An anemometer requires a certain amount of wind to overcome start-up friction. I have seen thresholds of 0.9 through 2.5 miles per hour for both anemometers and wind vanes.
Anemometers have a rotating mass which requires some amount of time to fully respond to the wind.
Typical distance constants are 8.9 feet through 20 feet on professional equipment. Since a windmill shouldn't be placed in an area where the winds won't produce enough power, the meteorology department did research, like prospecting. Good areas were usually rolling hills with low vegetation because power-producing windmills don't like gusts. However, even in gently rolling hills covered only with grass, the windspeeds and directions fluctuated. The highest sensors always had the smoothest readings, but they still varied.