Frost

Frost means the condition of temperature when below the freezing-point of water. The average temperature at any spot in the British Isles is for no part of the year below freezing-point (32° Fahr.), though exceptional circumstances may bring about a period of frost extending over a large area for several days in succession. Such weather is usually settled, with high barometer and little wind. The ground in a given locality may be warm, but the air above it below freezing-point. This is the case -when the growing warmth of early morning causes the heating of the layers of air nearest the ground, and a slow flow downwards of the colder upper layers. Conversely, the air may be above 32° Fahr. and the ground below that temperature, as when rapid radiation after sunset from the earth into a clear atmosphere above causes the earth to cool faster than the air. If there is but little water vapour in the air under sneli conditions, the dewpoint (q.v.) may be below 32° Fahr., and. that temperature being attained at night-time, dew will be deposited in the crystalline solid form. This is known as hoar-frost (q.v.Y