Hoar frost is a type of feathery frost that forms as a result of specific climatic conditions. The word ‘hoar’ comes from old English and refers to the old age appearance of the frost: the way the ice crystals form makes it look like white hair or a beard.
How does hoar frost form?
Ground frost forms when the air is still and cold, usually on clear winter nights. Water vapour in the air condenses on solid surfaces, and as the surface temperature drops below 0°C, ice crystals form.
Hoar frost requires slightly different conditions. It forms when the water vapour in the air comes into contact with solid surfaces that are already below freezing point. Ice crystals form immediately, and the ice continues to grow as more water vapour is frozen. On a still night, it can grow well on tree branches, where the surface temperature is unlikely to rise above zero for several hours.