fog

The word 'fog' first appeared in English in the mid-sixteenth century, surprisingly late for such a common atmospheric phenomenon.‌​‌​‍​‍​‌​‍​‌​‌​‍​‌​‍​‌​‍​‌​‌​‌​‌​‌​‍​‍​‍​‌​‌​‌​‍​‌​‍​‍​‍​‍​‍​‌ Before 'fog' arrived, English speakers used 'mist' (from Old English 'mist,' from Proto-Germanic *mihstaz) as the primary word for low-visibility atmospheric moisture. The origin of 'fog' is uncertain, but the most probable source is a Scandinavian word: Norwegian 'fog' means 'spray,' 'shower,' or 'snowstorm,' and Danish 'fog' means 'spray' or 'drift.' The word may have entered English through maritime contact with Scandinavian seafarers.

There is an older, unrelated English word 'fog' meaning 'long grass left standing after hay has been cut' or 'aftermath grass.' This word appears in Middle English from the fourteenth century and survives in dialectal use. Some etymologists have speculated that the atmospheric 'fog' might be connected — fog obscures the landscape as tall grass obscures the ground — but this connection is generally considered folk etymology rather than genuine derivation.

Meteorologkically, fog is defined as a cloud at ground level that reduces visibility to less than 1 kilometer. If visibility is between 1 and 2 kilometers, the condition is called 'mist.' Fog forms when air near the ground cools to its dew point, causing water vapor to condense into tiny suspended droplets. Several mechanisms produce fog: radiation fog forms on clear, calm nights when the ground radiates heat and cools rapidly; advection fog forms when warm, moist air moves over a cooler surface (the famous San Francisco fog is advection fog, caused by warm Pacific air flowing over the cold California Current); upslope fog forms when moist air is pushed up a slope and cools adiabatically; and steam fog forms when cold air moves over much warmer water.

Fog has had a profound impact on naval and maritime history. Before radar, fog was one of the greatest hazards of seafaring. The foghorn — a loud, low-pitched warning device — was developed in the nineteenth century to warn ships of coastlines and obstacles. The first steam-powered foghorn was installed at Partridge Island, New Brunswick, in 1859. The distinctive two-tone sound of a foghorn has become an iconic auditory symbol of the coast and maritime life.

London's relationship with fog defined the city's identity for centuries. The combination of natural Thames valley fog with coal smoke created the infamous London 'pea-soupers' — thick, yellow-green smogs that could reduce visibility to a few feet and persisted for days. Dickens wrote fog as a character in itself: the opening of 'Bleak House' (1853) is a celebrated description of London fog as a metaphor for the impenetrable confusion of the Court of Chancery. The 'London particular' — Dickens's term for the city's signature fog — became a widely recognized phrase.

The Great Smog of December 1952 was the catastrophic culmination of London's fog problem. A temperature inversion trapped coal smoke at ground level for five days. Visibility dropped to near zero. The smog was so thick that indoor performances at theaters and cinemas were cancelled because the audience could not see the stage or screen. An estimated 4,000 people died during the event, with later analyses suggesting the true death toll was 10,000–12,000. The disaster led directly to the Clean Air Act 1956, which restricted coal burning in urban areas and began the end of London's smoke-fog era.

Figuratively, 'fog' means confusion or obscurity: 'a fog of war,' 'brain fog,' 'fogging the issue.' The 'fog of war' — the uncertainty and confusion inherent in military operations — was popularized by the Prussian military theorist Carl von Clausewitz in 'On War' (1832), though he used the German 'Nebel des Krieges' (mist of war).