THE weather in recent weeks has conformed nicely to an old English commentary: First it rained and then it blew/ Then it hailed and then it snew/Then it friz and then it thew/And then it friz again anew.
Strictly speaking, it could be said to have friz, as the rhyme puts it, whenever the temperature falls to zero degrees Celsius or below. Indeed, frost may occur with no visible signs whatever of its presence - a so called black frost in which case the only way of detecting it with confidence may be with the aid of a thermometer.
But the term "frost" is also used to describe those white icy deposits common in freezing conditions deposits that can be seen and recognised for what they are without recourse to instruments of any kind.
White frost, or hoar frost, for example, is that white crystalline deposit often seen on grass and other outdoor surfaces on bright clear winter mornings. Indeed the term "hoar" is an old English word for "white".
It is closely related to dew, which occurs when water vapour in the air condenses on to a cool surface underneath; if the temperature of the underlying surface is below zero, the deposition is in the form of soft, white ice crystals, and the result is hoar frost.
On some occasions, indeed, there may already be a deposit of dew before the temperature falls below zero, in which case when the drops of dew subsequently freeze, the result is white dew, or silver frost.
Some surfaces are more susceptible to hoar frost than others, the key being the drop in temperature experienced by the surface in question. The family car parked near the house, for example, recoups some of the heat it loses by radiation from the energy radiating outwards from the brickwork of the building; the heat absorbed by the metal from this source retards its drop in temperature, and curbs the formation of hoar frost on adjacent portions of the car.
The net loss of heat by radiation from vertical surfaces, moreover, is less than that from horizontal ones, so vertical surfaces are less prone to troublesome deposits.
From many points of view, the most troublesome and dangerous form of frost is glazed frost. It occurs when rain falling from warmer air above freezes on striking the cold ground and forms a clear transparent sheet.
The result is similar to that which occurs when a layer of water already on the surface of a road, or a thin layer of moisture previously deposited as dew, freezes to produce the treacherous condition popularly known as black icef.