“I should be glad to hear your questions, and will answer them if I can.”
“I have noticed that dew falls on clear nights, but not very often on cloudy nights. I don’t see why that is so.”
“Have you ever noticed whether cloudy nights or clear nights are the warmer?”
“Cloudy nights are commonly warmer, I think, but I never could see the reason for that, either.”
“Can you tell why a newspaper spread over a tomato vine keeps the frost from the vine?”
“Because the frost comes upon the paper instead of the vine, of course.”
“But why do you say, of course? Why does not the dew—for frost is nothing but dew frozen as it forms—come upon the under side of the paper?”
“How could the dew fall upon the under side?”
“That is just the point which we need first of all to understand. Men commonly speak of dew as if it fell. I don’t know but I have spoken of the falling of dew in this lesson. But dew does not fall at all. The vapor simply touches some cold object, and is condensed upon it. The vapor by its elasticity presses against the cold body, and the process of condensation continues until either the body is warmed by the heat set free so that its temperature rises above the dew point, or till the vapor is so far exhausted that the dew point falls below the existing temperature. Dew is formed upon the upper surface and not upon the under, because the upper surface is cool and the under surface is warmer. Beneath the paper spread over the tomato vine, the earth is radiating heat and the paper is radiating it back again. If the paper were not there, the heat would be radiated into space and not returned again. The vine would soon radiate away its little store of heat, its temperature would sink, below the dew point, and dew or frost would be deposited upon it. The under surfaces of objects are kept warm by the radiation from the earth. In the same manner clouds are wrapped around the earth and keep it warm by radiating back its radiant heat. Dew is not formed on cloudy nights, because they are warmer: the clouds throw back the heat which otherwise would be lost in open space.”
“I never knew before,” said Peter, “that clouds were of any great use except to send down rain.”