A. Because any covering prevents the radiation of heat from the tree; and if the tree be not cooled down by radiation, the vapour of the air will not be frozen as it comes in contact with it.
Q. Why is the bass or canvass itself (which covers the tree) always drenched with dew?
A. The bass or canvass covering radiates heat both upwards and downwards; and is, therefore, so cooled down, that it readily chills all the vapour of the air (which passes over it) into dew.
Q. Why does snow at the foot of a hedge or wall melt sooner, than in an open field?
A. Because the hedge or wall radiates heat into the snow beneath, which melts it.
Q. Why is there no dew after a windy night?
A. 1st—Because the wind evaporates the moisture, as fast as it is deposited; and
2ndly—It disturbs the radiation of heat, and diminishes the deposition of dew thereby.
Q. Why are VALLEYS & HOLLOWS often thickly covered with dew, although they are sheltered?