A. Because, when the night is clear and fine, the surface of the ground radiates heat most freely; and (being cooled down by this loss of heat) chills the vapour of the air into dew.
Q. Why is there no dew on a dull cloudy night?
A. The clouds arrest the radiation of heat from the earth; and (as the heat cannot freely escape) the surface is not sufficiently cooled down to chill the vapour of the air into dew.
Q. Why is a cloudy night warmer than a fine one?
A. Because the clouds prevent the radiation of heat from the earth; and, therefore, the surface of the earth remains warmer on a dull cloudy night.
Q. Why is dew most abundant in situations most exposed?
A. Because the radiation of heat is not arrested by houses, trees, hedges, or any other thing.
Q. Why is there scarcely any dew under a shady tree?
A. The shady head of the tree both arrests the radiation of heat from the earth, and also radiates some of its own heat towards the earth; and, therefore, the ground (underneath a tree) is not sufficiently cooled down to chill the vapour of the air into dew.