A magical winterland has descended over much of the country since the new year, as thick frost has turned the landscape as white as snow. Much of this is hoar frost, an old English word meaning white-haired, old and venerable. It is a soft, feathery white layer of ice crystals that covers the ground, branches, wires and most other exposed objects. For hoar frost to form there needs to be clear skies, moist air and freezing temperatures. It also needs calm conditions, as Samuel Taylor Coleridge explained in Frost at Midnight (1798):

The Frost performs its secret ministry,
Unhelped by any wind.

As the ground loses its heat at night, the air grows colder until the moisture contained within it condenses and freezes on twigs, grass or any other exposed object. Once the ice has started to form, the water in the air will freeze directly on the object. This creates interlocking ice crystal needles that sparkle in sunlight as the smooth faces of the crystals reflect the light.

Another sort of frost seen in many places in the past few days is rime. This is a thick mass of white frost that can coat everything like a layer of icing sugar. Rime frost occurs under different conditions from hoar frost – it appears during cold, foggy nights.

Curiously, the water in fog can stay liquid well below freezing, but it instantly freezes on anything it touches below 0C (32F). If the freezing fog blows more droplets on to a tree or another large object the frost freezes on the wind-facing side of the tree, which gives it a surreal lopsided shape.