How thick is the thickest fog? © Getty Images

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.

Asked by: Nina Cunningham, Oxfordshire

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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. The Great Smog of 1952 was so bad that it penetrated indoor areas and drivers couldn’t see their own headlights!


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