How thick is the thickest fog?

Water droplets in the air can reduce visibility enough as it is, but add pollution to the mix and things get much worse.

18th April 2018
How thick is the thickest fog?

Asked by: Nina Cunningham, Oxfordshire

By definition, fog has a visibility of less than 1km, but it can get much thicker than that. The Met Office visibility scale runs down to a Category X fog, where visibility is less than 20m. If fog gets mixed with industrial pollution, it becomes smog and can be thicker still. During the Great Smog of 1952, drivers couldn’t see their own headlights! 

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