Frost Report

In my ignorance, I thought there was just 'frost' - I was aware of terms like 'ground frost' and 'hoar frost' and 'window frost', but I didn't realise there are significant differences. You can read the full details at the bottom of this post - or just watch this little clip from the Met Office and stun your friends with your weather knowledge [but be prepared for  their frosty or glazed expressions though]

Ground frost ; A ground frost refers to the formation of ice on the ground, objects or trees, whose surface have a temperature below the freezing point of water. During situations when the ground cools quicker than the air, a ground frost can occur without an air frost. 

Air frost; An air frost occurs when the air temperature falls to or below the freezing point of water. An air frost is usually defined as the air temperature being below freezing point of water at a height of at least one metre above the ground.

Hoar frost; From ‘hoary’, meaning aged and whitened, in reference to the shaggy and feathery coating that hoar frost leaves. It occurs under calm, cloudless skies, when there is little or no wind, and when cold air is trapped under warmer air.

Advection frost; Strong, cold winds prettily rim the edges of objects and plants with tiny spikes of frost, usually pointing in the direction of the wind.

Window frost; Also known poetically as fern frost or ice flowers, this is the frost that creeps across window frames forming swirls, feathers and other patterns, caused by the difference between the very cold air on the outside of the glass and the warmer, moderately moist air on the inside. The growth of the patterns responds to imperfections on the glass surface.

Glaze and rime; Frost is sometimes confused with glaze or rime. Rime is a rough white ice deposit which forms on vertical surfaces exposed to the wind. It is formed by supercooled water droplets of fog freezing on contact with a surface it drifts past. Glaze can only form when supercooled rain or drizzle comes into contact with the ground, or non-supercooled liquid may produce glaze if the ground is well below 0 °C. Glaze is a clear ice deposit that can be mistaken for a wet surface and can be highly dangerous.