A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to freeze the vapour condensed upon its surface.
Q. Why does hoar-frost very often cover the ground and trees, when the water of rivers is not frozen?
A. Hoar-frost is not the effect of cold in the air, but the cold of the earth (produced by excessive radiation); in consequence of which, the dew (condensed upon it) is frozen.
Q. Why is the hoar-frost upon grass and vegetables much thicker than that upon lofty trees?
A. Because the air (resting on the surface of the ground) is much colder after sun-set, than the air higher up; in consequence of which, more vapour is condensed and frozen there.
Q. Why is the air (resting on the surface of the earth) colder than that in the higher regions?
A. Because the earth radiates more heat than the leaves of lofty trees; and, therefore, condenses and freezes the vapour of the air more rapidly.
Q. Why are evergreens often frost-bitten, when lofty trees are not?
A. Evergreens do not rise far above the surface of the earth; and (as the air contiguous to the earth is much colder than that in the higher regions) therefore, the low evergreen is often frost-bitten, when the lofty tree is uninjured.
Q. Why are tomb-stones covered with hoar-frost, long after it has melted from every object around?