What is frost?
Frost occurs when the temperature of the air in contact with the ground is below the freezing-point of water (‘ground frost’ or ‘air frost’, respectively).
What are the different types of 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. A grass frost, an un-official type of ground frost, can occur when other surfaces - such as concrete or road surfaces - don't experience a frost, due to their better ability at holding onto any warmth. It is possible for a grass frost to occur in late spring or even early summer when the risk of more wide-spread frosts has disappeared and is something that gardeners in particular need to be aware of.
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 is composed of tiny ice crystals and is formed by the same process as dew, but when the temperature of the surface is below freezing point. The 'feathery' variety of hoar frost forms when the surface temperature reaches freezing point before dew begins to form on it. A 'white' frost, composed of more globular ice, occurs when the dew forms first, then subsequently freezes. The presence of fog tends to prevent the formation of hoar frost as it reduces the potential for radiational cooling of surfaces.
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.