Fog is a cloud at ground level that reduces horizontal visibility to less than 1,000 metres. It consists of tiny water droplets (or ice crystals in very cold conditions) suspended in air that has cooled to its dew point. When visibility is between 1,000 and 5,000 m, the condition is called mist. Fog is essentially a stratus cloud touching the ground.
Several types exist, classified by formation mechanism: radiation fog (forms on clear, calm nights as the ground radiates heat and cools the air above — common in valleys and river basins; burns off after sunrise), advection fog (warm, moist air moves over a cold surface — common on coasts when warm maritime air crosses cold ocean currents), upslope fog (moist air forced up terrain), frontal fog (rain falling through cold air beneath a warm front saturates the lower layer), and steam fog (cold air passing over warm water — the "Arctic sea smoke").
Fog has enormous impacts on transportation and safety: it is one of the leading causes of multi-vehicle accidents on motorways and forces airport closures and flight diversions. In Spain, the Ebro valley, the Castilian mesetas, and inland river valleys experience frequent winter radiation fog, sometimes persisting for days or weeks under strong inversions and anticyclonic conditions. This persistent fog — called niebla or boira (in Aragonese) — traps pollution and limits sunshine, creating a bleak, cold, grey environment very different from the sunny Spain of popular imagination. Fog dispersal research includes heating, downdraft blowing, and seeding techniques, though none are widely operational.