Fog is essentially a cloud at ground level that causes a reduction in visibility to less than 1000 metres.
It becomes a much more noticeable thick fog when the visibility drops below 180 m, while severe disruption to transport occurs when the visibility falls below 50 m over a wide area (referred to as dense fog)
What causes fog?
Fog is caused by tiny water droplets suspended in the air. The thickest fogs tend to occur in industrial areas where there are more pollution particles in the air allowing water droplets to coalesce and grow.