Why do people refer to heavy fog as "pea soup?"

Answer by  CarolynOsborne (2804)

The "pea soup" nickname for fog came from 19th century London which had terribly thick fog not just due to weather but also due to the enormous amount of coal smoke in the air. Pea soup is or was a common food and it is thick. Reducing coal emissions reduced the incidence of pea soup fogs.

Answer by  joantheresa (1421)

Fog caused by a mixture of dirt and smoke is a yellow smog and very thick, compared to 'pea soup.' The term originated in London in the 20th century.