Fog

Fog is a cloud at ground level made of tiny water droplets, or sometimes ice crystals, that reduces horizontal visibility. It forms when air near the surface becomes saturated, for example during night cooling, over moist ground, or when warm air moves over a colder surface. For pilots, fog can quickly make take-off, landing, and visual navigation difficult or impossible. Check METARs, TAFs, and visibility trends carefully, and delay or divert if the required visual references are not available.