I put a barn fan attached to the roof of my run. It kicks on when the temp in the run reaches 80° as measured by a temp/humidity sensor (white thing hanging on the support beam near the coop).
Since it points straight down, it's easy for the chickens to move out of the breeze if they desire. They seem to prefer to dig holes and lay on the cooler earth near the edges of where the air flow is. I do have the fan set to the lowest speed but it still moves a lot of air.
