Hoarfrost is a thin white layer of ice that forms on things outside when it is very cold — on grass, car windows, gates, and the twigs of trees. It looks as if the world has been dusted with sugar, or as if the grass has grown white hair. That 'white hair' idea is in the name: hoar is an old word for grey-white, the colour of old hair. Though actually it is not snow, and it is not quite the same as frost from frozen dew. Hoarfrost grows as tiny ice crystals out of the water in the air, on surfaces that are already below freezing. You see it on still, freezing mornings. The word sounds a little countryside or bookish; a lot of people just say frost. Weather people still like hoarfrost for the pretty, feathery kind. You may also see hoar frost (two words) or hoar-frost.