"One windmill will not put out much noise, but when you put a cluster of them together and get the blades all spinning at the same rate, then it begins to emit noise -- enough that our radar can 'hear' the noise." You could tell the radar to ignore that frequency, but if there's a thunderstorm with the same frequency, it also would be ignored. "The use of wind energy has become a big problem not only in the meteorological world, but also to the military community as airplanes could fly low under a wind farm and one would never know the difference as to whether they are looking at false returns from the wind farm or an enemy plane."