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