The frost fairies came

2011.01.11

There are three types of frost that occur in different ways:
Rime is ice formed when a damp, icy wind blows over flowers, branches and other surfaces. Rime frost looks like icing around the edge of petals and leaves, and only occurs when the temperatures are very low.
Hoar frost occurs when water vapour touches a very cold surface and freezes on it instantly. This can happen to the leaves and branches of plants, and will cover them with ice crystals that look like spiky fingers. It can also occur on other freezing surfaces such as soil and metal, and so can often be seen on cars. Hoar frost can occur at higher temperatures than rime frost – usually when the air temperature is around 0°C (32°F). However, the ground is usually much colder, and the air must be moist for the ice crystals to form.
Fern frost. In particularly cold weather, fern frost may appear on windows. This happens when tiny water droplets (dew) first form on the cold glass. These then turn into ice and more moisture freezes on top. As this process continues, more ice crystals are formed and the frost develops into what looks like feathery fingers. Fern frost can create beautiful patterns of ice crystals, which often look leaf or fern like – hence the name.