Because the clouds radiate back to the earth the heat which the earth evolves?
Because, also, the clouds radiate to the earth the heat they have derived from the solar rays during a cloudy day.
398. Why is the earth warmer than the air during sunshine?
Because the earth freely absorbs the heat of the solar rays; but the air derives comparatively little heat from the same source.
399. Why does the earth become colder than the air after sunset?
Because the earth parts with its heat freely by radiation; but the air does not.
400. Why do glasses, mats, or screens, prevent the frost from hitting plants?
Because they prevent the radiation of heat from the plants, and also from the earth beneath them.
401. Why are the screens frequently covered with dew on their exposed sides?
Because they radiate heat from both their surfaces. A piece of glass, laid horizontally over the earth, would radiate heat both upwards and downwards. But on its lower surface it would receive the radiated heat of the earth, while from its upper surface it would throw off its own heat and become cool. Therefore dew would be deposited upon the upper, but not on the under surface.