# Pea soup fog
**Pea soup fog**, also called a **pea-souper**, black fog, or killer fog, is a very thick fog, usually yellowish, greenish, or blackish, formed when natural fog mixes with soot and sulfur dioxide from burning coal[1](https://weather.com/news/weather/news/2023-11-02-weather-words-pea-soup-fog). The name is most closely tied to the coal-smoke fogs of London from the mid-17th century to the 1960s, and it lives on as an informal word for any dense fog, especially in Canada[1](https://weather.com/news/weather/news/2023-11-02-weather-words-pea-soup-fog) • [2](https://dchp.arts.ubc.ca/entries/peasoup%20%28%282%29%29).
| Key fact | Detail |
|---|---|
| What it is | Polluted fog colored by soot and sulfur dioxide from soft coal burned in homes and industry[1](https://weather.com/news/weather/news/2023-11-02-weather-words-pea-soup-fog) |
| Other names | Pea-souper, black fog, killer fog; in London, "London particular"[1](https://weather.com/news/weather/news/2023-11-02-weather-words-pea-soup-fog) |
| Name's origin | Soup of dried split peas, a yellowish-brown staple of London's poor[3](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/) |
| First dictionary citation | Herman Melville, dated 1849 in the Oxford English Dictionary, though other sources date it to 1850[3](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/) |
| Worst episode | The Great Smog of London, 5 to 9 December 1952[4](https://www.rmets.org/metmatters/65-years-great-smog) |
| Legal response | Clean Air Act 1956, which created smoke-control zones[4](https://www.rmets.org/metmatters/65-years-great-smog) |
## Formation and color
Fog is a cloud at ground level that cuts visibility below 1,000 meters, and the UK Met Office calls it dense when visibility falls below 100 meters[5](https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog). The thickest fogs tend to form in industrial areas, where pollution particles help water droplets grow[5](https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog). In historic London, smoke from thousands of chimneys burning soft bituminous sea-coal combined with natural fog to make what was later called smog, a word a Londoner coined in 1905[6](https://www.epa.gov/archive/epa/aboutepa/londons-historic-pea-soupers.html). Cold, still, high-pressure weather let these pollutants collect near the ground, and temperature inversions trapped the polluted air in the river basin, which is hemmed in by hills to the north[4](https://www.rmets.org/metmatters/65-years-great-smog) • [7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite).
**Why yellow.** Early London fogs were yellowish because of their sulfur content; by the 18th and 19th centuries they were often dun or black, darkened by soot mainly from domestic coal fires[3](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/). The [Royal Meteorological Society](https://www.edgepedia.org/royal-meteorological-society) describes the 1952 smog as a yellowish-green "pea-souper" in color, from soot and sulfur dioxide[4](https://www.rmets.org/metmatters/65-years-great-smog).
**The chemistry.** A 2016 study in the *Proceedings of the National Academy of Sciences*, led by Renyi Zhang of Texas A&M University, found that sulfur dioxide in the 1952 fog was converted to sulfuric acid inside fog droplets, a reaction aided by nitrogen dioxide, another product of coal burning[8](https://www.sciencedaily.com/releases/2016/11/161115091556.htm). As the fog evaporated it left smaller acidic haze particles over the city[8](https://www.sciencedaily.com/releases/2016/11/161115091556.htm). The team found similar sulfate chemistry in modern Chinese haze, which begins from much smaller nanoparticles and, because of ammonia, is basically neutral rather than acidic like the London fog[8](https://www.sciencedaily.com/releases/2016/11/161115091556.htm).
## The term
The name refers to soup made from dried split peas, which gives a yellowish-brown color and was a staple of the city's poor[3](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/). Sources differ on its first written use: the *New York Review of Books* reports that the *Oxford English Dictionary* first records it for 1849, citing Melville, while the *London Review of Books* dates Melville's phrase, "the old-fashioned pea soup London fog," to 1850 and reads it as already well-worn[3](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/) • [7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite). The related nickname "London particular" appears in the caption of an 1827 cartoon by Michael Egerton, and Dickens later used it in *Bleak House*[7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite).
Longman labels "peasouper" British and old-fashioned[9](https://www.ldoceonline.com/dictionary/peasouper). The *Dictionary of Canadianisms on Historical Principles* treats "peasoup" as a British survival in [Canadian English](https://www.edgepedia.org/canadian-english), with the adjective sense "extremely foggy" attested from 1880 and the noun "a dense, yellowish fog" from 1944; its 2015 web-frequency check found the term most common in Canada and least common in the United Kingdom[2](https://dchp.arts.ubc.ca/entries/peasoup%20%28%282%29%29).
## London's fogs
London switched from wood to coal centuries before the term arose, and Edward I banned the burning of sea-coal in 1272 without stopping it[6](https://www.epa.gov/archive/epa/aboutepa/londons-historic-pea-soupers.html). In 1661 John Evelyn's *Fumifugium* urged the King and Parliament to act on the smoke[6](https://www.epa.gov/archive/epa/aboutepa/londons-historic-pea-soupers.html). By 1880 the city had about 3.5 million coal fireplaces, and foggy days averaged 55 a year in 1881 to 1885 and 63 in 1886 to 1890[7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite). A fog in 1873 caused 268 deaths from bronchitis, and the fog of 1879 lasted from November to March[6](https://www.epa.gov/archive/epa/aboutepa/londons-historic-pea-soupers.html). Foggy days fell to about 20 a year by 1900, helped by gas cookers, electric motors, better grates, and enforcement, though dense fogs returned in 1921, 1922, and 1924[7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite).
**The Great Smog of 1952.** From 5 to 9 December 1952, smoke from coal burned for home heating, with pollution carried in from industrial areas on the European continent, cut visibility in London to a few meters[4](https://www.rmets.org/metmatters/65-years-great-smog). About 4,000 deaths were estimated at the time; later estimates reach as high as 12,000, with around 100,000 more people made ill[4](https://www.rmets.org/metmatters/65-years-great-smog). The [Clean Air Act 1956](https://www.edgepedia.org/clean-air-act-1956) followed, setting up smoke-control zones, restricting dark smoke emissions, and moving household heating toward cleaner fuels[4](https://www.rmets.org/metmatters/65-years-great-smog). A fog in December 1962, with more sulfur dioxide but less soot than before, killed about 750 people; Neal Ascherson, writing in the *London Review of Books*, calls it the last classic London fog[7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite). From the late 1960s, vehicle emissions replaced coal smoke as the city's main air-pollution threat[7](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite).
## References
1. [Weather Words: Pea Soup Fog, The Weather Channel](https://weather.com/news/weather/news/2023-11-02-weather-words-pea-soup-fog)
2. [peasoup, Dictionary of Canadianisms on Historical Principles, University of British Columbia](https://dchp.arts.ubc.ca/entries/peasoup%20%28%282%29%29)
3. [City in the Dark (review of Christine L. Corton, London Fog: The Biography), The New York Review of Books](https://www.nybooks.com/articles/2017/02/09/london-fog-city-in-the-dark/)
4. [65 years since the Great Smog of 1952, Royal Meteorological Society](https://www.rmets.org/metmatters/65-years-great-smog)
5. [Fog, Met Office](https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog)
6. [London's Historic "Pea-Soupers", US Environmental Protection Agency](https://www.epa.gov/archive/epa/aboutepa/londons-historic-pea-soupers.html)
7. [Brown Goo like Marmite (review of Christine Corton, London Fog: The Biography), London Review of Books](https://www.lrb.co.uk/v37/n19/neal-ascherson/brown-goo-like-marmite)
8. [London fog to Chinese haze: Sulfate formation, ScienceDaily](https://www.sciencedaily.com/releases/2016/11/161115091556.htm)
9. [peasouper, Longman Dictionary of Contemporary English](https://www.ldoceonline.com/dictionary/peasouper)
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*Topic: Encyclopedia › Physical world and mathematics › Earth sciences › Climate and weather › Meteorology and atmospheric science › Clouds › Fog and mist*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 25, 2026 · Edited: Sep 30, 2026; Oct 25, 2026 · Last review: Oct 25, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license