1 adj. — informal
extremely foggy.
Type: 2. Preservation — Pea soup or peasouper, relating to a thick fog, is a preservation from British English, with adjectival uses documented more than six decades before nominal ones (see meaning 2).
London, England, in particular, became notorious as its population surged during the Industrial Revolution for heavy, yellow fogs in autumn caused by coal fires. Though OED-3 (s.v. "pea-souper" (1)) marks peasouper as a British colloquialism, internet search results indicate the term is currently most frequent in Canada, closely followed by several other former British colonies, with frequency actually being the lowest in the UK (see Chart 1).
See also:
- 1880  [In her room -- bedroom and sitting-room in one -- Jemima Ann leans out of the little window and tries to catch a breath of air, where air in this pea-soup atmosphere there is none.] 
- 1899  In spite of the pea-soup fog which envelopes this city in a fetidity of sticky, unpudled heat, the yacht races are the only thing talked about. 
- 1934  Mr. Peisson appears to possess a thorough technical knowledge of ocean-going liners; and his description of the mad race through a "pea-souper" fog off Newfoundland revives, uncannily, memories of the ill-fated Titanic. 
- 1988  Off camera, a lot of work goes into maintaining the illusion. A pea-soup fog billows from a smoke machine along one hall, softening the background for the video camera. 
- 1998  Originally scheduled for Friday, poor weather conditions put the race in doubt altogether. "[The hill] was just completely socked in - total pea soup fog," Fawcett said. 
- 2015  An air inversion is to blame for conditions that stranded passengers and delayed flights up and down coastal B.C. Nanaimo Airport fared better than some Island airports, but not without some flight delays from the pea soup conditions. 
2 n. — informal
a dense, yellowish fog.
Type: 2. Preservation — Attested adjectival uses (see meaning 1) are older than the nominal uses shown here by about six decades. It seems possible, in contrast to our assessment of the same scenario in peasoup ((1)), that the nominal function is, in this meaning, younger than the adjectival one.
- 1944  In a pea souper that would have just about tasked all the neon lighting power in Canada to penetrate, and one that many times obliterated everything beyond a bare outline of the infield, our Sunday Classers scored a misty 4-1 win over Vancouver Neons to wind up the East-West series here at one game apiece. 
- 1953  Max Bentley whose future as a Maple Leaf has been rather cloudy ever since he darted out of town in a hurry last spring, is in a real pea-souper now. 
- 1964  The pea-souper forced the plane of President Lyndon Johnson to shift its landing from Kennedy Airport on Long Island to Newark, NJ, when Mr. Johnson flew from Washington to attend the funeral of the New York City mayor's wife, Mrs. Robert F. Wagner. 
- 1980  ''People were going down the highway about 60 miles an hour,'' Mr. McCallum said. ''It was suicidal.'' He said visibility was very limited in low spots. ''I couldn't see 50 feet. It was thick fog - pea soup." 
- 2005  That the true culprit blocking the military jumpers was a sudden and heavy fog was, of course, obvious to all. But that on its own gave cause to panic. Was this evening of CFL history, the first exhibition game ever staged in Nova Scotia, about to be ruined by an impenetrable pea soup? 
Images:
Chart 1: Internet Domain Search, 7 May 2015