412. Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?
Because white, or hoar frosts, result from the coldness of the earth, which, from its great radiating power, is always varying. But black-frosts result from the coldness of the air, which is liable to less variation of temperature than the earth.
A black-frost results from the coldness of the atmosphere, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to the frozen surface of water.
414. Why are black-frosts said to last?
Because as they result from the temperature of the air, which is less likely to vary than that of the earth, there is a probability that the coldness thereof will last for some time.
"And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground,"—Judges vi.