Fog Facts



  • Fog spreads slowly upwards from the surface of the water seen here over the Ganges in India.
  • Like clouds, mist is billions of tiny water droplets floating on the air. Fog forms near the ground.
  • Mist forms when the air cools to the point where the water vapour it contains condenses to water.
  • Meteorologists define fog as a mist that reduces visibility to less than 1 km.
  • There are four main kinds of fog: radiation , advection, frontal and upslope.
  • Radiation fog forms on cold, clear, calm nights. The ground loses heat that it absorbed during the day, and so cools the air above.
  • Smog is a thick fog made when fog combines with polluted air.
  • Advection fog forms when warm, moist air flows over a cold surface. This cools the air so much that the moisture it contains condenses.
  • Sea fog is advection fog that forms as warm air flows out over cool coastal waters and lakes.
  • Frontal fog forms along fronts.
  • Upslope fog forms when warm, moist air rises up a mountain and cools.