What does fog mean?
Fog is a weather phenomenon: a cloud of minute water droplets hanging in the air at or near ground level, thick enough to reduce visibility to less than a kilometre. It forms when air cools to its dew point, most often over calm land or water in the early morning or late evening. In ordinary speech the word also carries a rich figurative life — people speak of 'brain fog' when thinking feels dulled, or of a 'fog' of confusion surrounding an event. Meteorologists distinguish fog from mist by density, while photographers use 'fogging' to describe a hazy flaw in an image. The word connotes obscurity, uncertainty, and concealment, appearing everywhere from shipping forecasts to detective fiction. Its long history — from London's notorious coal-smoke fogs to modern weather warnings — has made it one of English's most evocatively atmospheric words.
A visible aerosol of tiny water droplets condensed in the air at or near the surface, reducing visibility.
"Dense fog rolled in off the sea, forcing the ferry to slow to a crawl."
Meteorological definition requires visibility below one kilometre.
To cover, obscure, or become covered with fog; also (in photography) to blur or cloud an image.
"Condensation fogged the bathroom mirror within minutes."
The plural is standard but uncommon, since fog is usually treated as an indefinite mass; plurals appear mainly when referring to distinct fog events or regions, e.g. 'the fogs of autumn'.
"The shipping forecast warned of fogs along the northern coast."
London's infamous 'pea-soupers' were so thick that pedestrians once walked the streets led by 'link boys' carrying lanterns — until the Clean Air Act of 1956 cleared them for good.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fog
The noun 'fog' is generally traced to Old Norse, where related words meant spray, drizzle, or long wet grass, and it appears in English dialect use before spreading into general use in the 16th century; earlier English usage applied 'fog' chiefly to rank, unseasonable grass rather than weather. Its exact earliest attestation is debated, so a broad description is safest. The word's association with obscuring vapour became firmly established by the early modern period, especially in maritime and London contexts.
Related word forms
How fog is actually used
Fog is common in both everyday and technical contexts. Meteorologically, it is distinguished from mist strictly by visibility distance (under one kilometre). The figurative sense 'fog of confusion' or 'brain fog' is widespread in modern informal usage, especially in discussions of illness, fatigue, and menopause.
Easily confused with fog
Mist is thinner and reduces visibility less severely than fog, which is conventionally defined as reducing visibility below one kilometre.