The National Weather Service runs a network of Doppler radar stations intended to map storms by detecting moving raindrops.
Serendipitously, windmill blades being mounted well above the ground and containing conductive lightning arresters produce very strong radar echoes. When the blades are turning, the radar echoes are Doppler-shifted and therefore show up on Doppler radar maps as stationary targets, brighter than all but the most severe thunderstorms or tornadoes.
Of course, when the wind isn’t blowing, windmills that aren’t in tourist-attraction areas will quit turning, and hence will disappear from the Doppler radar maps. This makes those weather maps a convenient graphic aid to estimating the amount of electricity being generated by wind farms.
Texas is widely known as the state having, by far, the largest installed wind-generating capacity. Naturally, Texas also has the largest number of Doppler radars. Its maps are available online, so it is possible for any interested person to keep track of how often that immense wind-generating capacity is sitting idle.
Clearly, the good people of Texas have been as badly deceived by exaggerated claims of abundant clean wind energy as we in Washington and Oregon.