Fog
Clouds & visibility meteorological fog
Fog is a cloud at ground level: tiny suspended droplets, or less often ice crystals, reduce visibility below one kilometre.
This time the cloud is down here
Fog is not visible water vapour; vapour itself is invisible. What we see is tiny droplets or ice crystals. They scatter light and hide distant objects. Meteorologists call this fog below 1,000 metres visibility when these suspended particles cause the reduction.
How fog forms
- Night-time cooling: Under clear skies, the ground cools and the air above it may reach its dew point.
- Moist air over a cold surface: Cooling from below can bring the air to saturation.
- Evaporation: Warmer water can add vapour to cold air above it.
Why fog sometimes stays
Sunshine can warm the air and evaporate the droplets. Wind may mix or carry away moist air. Under an inversion, however, fog may linger. A grey morning does not automatically mean a grey day.
Fog can be very local
Banks may develop in hollows and damp low ground while nearby roads remain clear. Below freezing, supercooled droplets can freeze onto objects and form rime. An area forecast therefore cannot replace checking actual visibility and surface conditions where you are.