I have never felt the wind reduce around a wind farm before. Each wind turbine is like needle in a haystack of wind, I can’t imagine anything actually measurable.
Have you actually done some formal measurement in front of and behind the wind farm?
Because the way you're writing suggests not.
Btw turbines do slow down the wind and it is measurable, turbines at the back produce less energy than those at the front, wind farms are laid out specifically to try and avoid this.
Sure, if the turbines are spaced too closely they will lose efficiency. But as far as the surrounding environment is considered, existing wind farms don’t have any noticeable effects on the wind. Perhaps when we start building bigger more consequential wind farms we will notice something.
