The threshing floors were and still are under the open air, and usually circular. The second sign Judges 6:40, would be more convincing than the former, because it is the nature of fleeces to attract and retain moisture.
Related Commentaries of Judges 6:37
Judges 6:37
37 Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel by mine hand, as thou hast said.