“Interference with ATC radar is not a serious issue, just more NIMBY pettifogging. (Weather radar is another issue.)”
The Twin Groves wind farm east of Bloomington, Illinois shows up on the Lincoln, Il weather radar on fairly rare occasions. It can be identified on the time lapse radar by a stationary green, either with no other weather action or with storm systems moving by. It probably fouls up radar derived rainfall totals.
My guess is that it is caused by moisture on the blades.
I refrained from mentioning weather radar. Interference with ATC radar is a bad joke.
Weather radar, otoh, is a different issue. Weather radar are much more sensitive than ATC radar. They take long dwells with very high Doppler resolution and attempt to filter out ground clutter. Typically, precipation has a velocity spread of a few meters per second, so even in places where average range rate (and hence Doppler) is very low, there is a reasonable signal and they infer the mean reflectivity from the available clutter free spectrum.
Regardless of whether or not the blades are wet, they will look huge compared to moderate to light rainfall and their Doppler spectrum will appear white, it will fill all Doppler bins. Returns can enter the radar through antenna sidelobes or by refraction under “favorable” conditions.
Many ATC radar have ancillary weather channels. They use the same pulses as the ATC radar, but have separate receivers and signal processing optimized for weather processing. Since these radars are optimized for the ATC mission, they don’t make very good weather radars, regardless, hence things like TDR (Terminal Doppler Radar, basically, windshear monitors.) I suspect that any ATC radar with those towers in their field of view will be blind to weather at the same range-azimuth coordinates.