The official definition of fog is a visibility of less than 1,000 m. This limit is appropriate for aviation purposes, but for the general public and motorists an upper limit of 200 m is more realistic. Severe disruption to transport occurs when the visibility falls below 50 m. Useful labels for these three categories are aviation fog, thick fog and dense fog. The reduction in visibility is due to tiny water droplets suspended in the air. The thickest fog tends to occur in industrial areas where there are many pollution particles on which water droplets can grow. The Met Office...