What is freezing fog?

Warnham West Sussex
Warnham, West Sussex Credit: Wayleebird

During the colder months we sometimes hear the term 'freezing fog' in weather broadcasts, but what exactly is it?

Freezing fog forms in exactly the same way as our everyday fog. Long, clear nights and calm conditions allow the temperature of the ground to drop, cooling the air above it and leading to the formation of tiny water droplets, resulting in fog. Fog becomes 'freezing' when the temperature of the air falls below zero Celsius. Fog in itself is a hazard but freezing fog is notoriously stubborn and can often last all day.

Freezing fog and rime at Stockbridge Down Credit: Jim Reilly

Despite this is can also create some really beautiful scenes courtesy of a phenomenon known as 'rime'. The droplets in freezing fog remain as liquid despite being sub-zero, these are known as super-cooled water droplets. As soon as they make contact with a surface (such as a branch or railing) they freeze immediately creating icy deposits, known as rime, on the vertical, windward side of objects. The effect can be stunning.

Rime at Windmill Hill Credit: Ginny Boxall