Albert Barnes Commentary Judges 6:37

"behold, I will put a fleece of wool on the threshing-floor; if there be dew on the fleece only, and it be dry upon all the ground, then shall I know that thou wilt save Israel by my hand, as thou hast spoken." — Judges 6:37 (ASV)

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.