What is fog and what causes it?

Fog is formed when the earth's surface, or near to it, becomes saturated with water vapour by any of 3 processes:

• cooling

• addition of moisture

• mixing with another air parcel

Fog is essentially a cloud at ground level, that causes visibility less than 1000 metres. AIthough it is often only reported as fog at less than 180 metres visibility.

Mist is the result of the suspension of water droplets in the air, but at a lower density, so the visibility is slightly decreased but is still greater than 1000 metres.

Haze is the suspension of extremely small, dry particles in the air, not water droplets.

Coastal Fog

Coastal fog refers to fog over coastal areas. It is also known as a Sea Fret on the English east Coast or a Haar in eastern Scotland. This is usually advection fog, meaning it forms when relatively warm air passes over a cool surface, such as warm air moving over the cold North Sea towards the UK's east coast. This type of cooling means the water vapour condenses out of the warm air, forming into tiny water particles. It is more common in spring and summer when the air warms faster than the sea does. Its formation is affected by wind strength, wind direction and land temperature. Winds blowing in from the east can cause blankets of fog, which can linger a long time if the land is cold.