Fog
Fog is essentially a cloud at ground level that causes a reduction in visibility to less than 1000 metres.
What causes fog?
Fog, like clouds, is caused by tiny water droplets suspended in the air, but not all fog forms in the same way and there are a few different types of fog.
Dense fog is when the visibility falls below 100 metres which can cause severe travel disruption. 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.