Page:Handbook of Meteorology.djvu/77

 Rh also may prevent radiation. For the foregoing reasons the problems concerning the formation of dew are of much importance.

Frost.—If the temperature of the air is below freezing, the water vapor will be deposited in the form of minute crystals of ice, which reflect the light in such a manner that a silvery appearance results—the hoar frost of popular tradition. In some instances the moisture is doubtless deposited as dew, which afterwards is frozen. Sometimes, too, partly melted frost or slowly freezing dew forms a glazed and semi-transparent coating—the rime of tradition. From the nature of the case, rime is more hurtful to vegetation than is hoar frost.

When rain freezes as it falls on leaves, stalks, and twigs the ice varnish is the traditional black frost. Strictly speaking, it is not frost at all. It is a freezing which involves the surface of the vegetation. The superficial juices of the plant are frozen, to the extent that the cells of the plant are ruptured.

Hoar frost injures tender plants but does not necessarily kill them. Black frost, on the other hand, is apt to kill tender plants and to injure many hardy plants. A temperature as low as 25°, without frost, may be as fatal to tender plants as a black frost.

Warnings of late spring and early fall frosts are sent out from Weather Bureau stations. Close observation, however, will enable one to foretell a possible frost by watching the temperature and humidity. When the air is still, the humidity high, and the sky clear, frost may be expected if the temperature at sunset is 40° or lower; indeed, under such conditions the temperature is likely to fall to the freezing point by 2 o’clock on the following morning and to remain below freezing for a short time after sunrise.

The greater likelihood of frost in low spots, such as valley floors, as compared with the higher levels of the adjacent slopes, is an important factor in fruit farming and, in fruit-growing regions, pretty accurate surveys have been made of the lands likely to be visited by killing frosts. Nevertheless, by far the greater area of tender crops is within the region of killing frosts; hence the necessity of making use of all available knowledge in the matter.