As the blades spin, the radar systems may be swamped with Doppler-shifted reflections, or may pick up each individual blade, causing the farm to appear as a confusing mass on the screen, making it hard to identify aircraft or other targets, or obscuring important data from the radar screen. The result of this interference is to cause radar "black holes" in which aircraft moving of any height cannot be distinguished and so cannot fly safely. In addition, wind farms can create a ‘shadow’ in their wake in which low flying aircraft cannot be distinguished. Wind farms, especially large ones, may also interfere with the onboard navigational aids of ships and aircraft. This is of particular concern in regard to the very large offshore wind farms which are currently being developed in Northern Europe.
Although the interactions are complex, in general wind turbines reflect radio waves in a number of ways. The Glass Reinforced Plastic that makes up the majority of wind turbine blades is itself reflective, as are the metallic lightning conductors which snake through the inside of the blades and towers. The towers and nacelles themselves may also have large radar cross sections, particularly when there are many turbines located together (in comparison to their length, cylinders are said to have a large RCS). Other factors such as the pitch and yaw of the turbine and the individual pitch of the blades may also affect the RCS. Blades spinning to and away from the path of the radar system at a 90° angle to the receiver will have a greater impact than those in other positions as a result of the enhanced Doppler effects.
The NOAA radar Ops Center has some very good data...
Radar Operations Center - Wind Farm Index
Wind Farm Impact On Doppler Radar