What is fog – and why does it occur?


You may have noticed that fog is the predominant weather type across parts of the UK today and over the past few days. With all the coverage in the media, you may be wondering what fog is and how it is formed. Put simply, fog is just cloud at the ground! More specifically, it is made of small water droplets suspended in the air just above the ground, which cause a reduction in the near surface visibility.

The water droplets form because the relative humidity has exceeded 100%, and the air can no longer hold water in the vapour form. It is therefore forced to condense out as liquid (or ice) droplets.

High values of relative humidity are achieved either by cooling the air or adding moisture to it. The reason such high values of relative humidity have been created often gives rise to names describing the type of fog which has formed. For example:

Radiation fog

This occurs under clear skies and calm conditions. The land surface emits thermal radiation into space and cools, forcing the near surface air to cool. If the cooling is sufficient, fog will form.

Advection fog

This occurs when moist air passes over a cool surface and is cooled. For example this can happen in coastal regions in the morning, when air is blown from the warm sea onto the cold land.

Evaporation fog

This occurs when cold air blows over warm water, which provides a source of moisture to the air. For example this can occur over warm lakes or swimming pools, where the fog appears like steam rising from the water surface.

Traditionally, “aviation fog” has been defined as a visibility falling below 1000m, although many airports (e.g. Heathrow) now do not start adjusting their procedures until visibility falls below 600m. For the general public, fog often does not start causing disruption until the visibility falls below 200m.

Ian Boutle – Senior Scientist.

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