By Philip Eden
The cold weather of the last two weeks has provided some excellent opportunities to observe the development of white frost.
It is important to distinguish between the two main types of white frost: hoar and rime. Hoar frost comprises ice crystals, often in the form of feathers or needles, which are seen on surfaces cooled by outgoing radiation - that is, the loss of heat energy to outer space which has its greatest effect at night. The frost may be composed in part of drops of dew frozen after formation, and in part by ice crystals deposited directly from water vapour in the atmosphere when the temperature is below freezing. This process is called sublimation.
Some materials cool more quickly than others while objects close to the ground cool more quickly than those higher up. The metal and glass of your car are very efficient 'radiators', so they are among the first things to develop a coating of frost. Blades of grass become frosted more quickly than concrete or asphalt because they are not significantly affected by the conduction of heat from below ground, whereas hard surfaces are. Outgoing radiation is inhibited by cloud and fog, while the collection of cold air in the lowest few feet of the atmosphere is easily dispersed by even a light wind. By contrast, the higher the humidity of the air is, the more water vapour is available for sublimation into ice crystals.
Thus the heaviness of the frost is not an indication of how cold it has been. Quite the contrary, in fact. The most copious deposits of hoar frost occur under cloudless skies with no wind when the air temperature is close to 0°C and the relative humidity near 100 per cent, particularly when these conditions persist for, say, 12 hours or more.
Rime is deposited by fog. Even when the temperature is several degrees below freezing, fog is comprised of tiny water droplets; they remain liquid mainly because of the surface tension of each droplet. But these droplets freeze immediately when they come into contact with other objects. Such a freezing fog will deposit rime on everything - trees, fencing, aerials, telephone lines. Again, the abundance of the deposit is not related to the temperature. When there is a wind, rime builds up on windward surfaces, and it can become several centimetres during a long-lasting fog with a steady breeze.
© Philip Eden