409. Why does the humidity of the atmosphere sometimes form clouds, and at others form fogs, mists, dews, &c.?
The result depends upon the varying temperature, motion, and direction of the atmosphere.
A warm light atmosphere, of a few day's duration, will elevate the vapours to the region where they are formed into clouds.
A chill air, lying upon the surface of the warmer earth, will occasion mists or fogs.
A cold earth, acting upon the vapours contained in a warmer atmosphere, will condense them and occasion dews.
410. Why are frosty mornings usually clear?
Because, in the cold atmosphere which preceded the frost, there was but little evaporation; and now that the frost has set in, the vapours that existed have become frozen in the form of hoar-frost.
411. Why are clear nights usually cold?
Because the "screen" afforded by the clouds does not exist; therefore the heat of the earth escapes, while the vapours of the air are abstracted from it by condensation into dew, thereby imparting great clearness to the nights.