From the second half of the 19th century until well around the 1950s England and especially London were associated with foggy, damp environments where nobody could see anything in the mist, except shadowy figures. This Ominous Fog and Mysterious Mist made it ideal for setting in which detectives are solving murder mysteries in near darkness.
Truth in Television for many decades. London fog was often so thick it was nicknamed pea soup, because of the greenish tinge. If you could see 10 feet in front of you in those days you were lucky; there are even reports of people falling into the Thames as a result, and sometimes the air and smell was so bad that people wore mouth masks. As you might expect, this wasn't really true "fog" (which, as an Englishman in San Francisco once commented, while it can move quickly, but is rarely actually that bad for visibility or smell), but rather a product of both fog and air pollution—that is to say, smog (="smoke"+"fog").
London, lying as it does in a river valley by a big river and very close to where it turns into an estuary, is rather humid and susceptible to actual fog, but the real concern comes when the smoke from all the coal fires in London up until the middle of the 20th century mixes with the fog, you get a new substance that you can't see through and smells vile. Domestic coal fires were the major contributory factor; Londoners had burned coal to heat their homes and businesses since at least the 17th century (there are plenty of reports from that time of how horrid the London air is from all the smoke), but the problem got especially bad after World War II, when all the good quality coal was being sold for export to try and pay off the war debt, leaving Britons to burn the low quality, sulphurous coal sold cheaply onto the domestic market. Another reason for the fog's existence were the factory smokes, which provided an awful smell too.
The toxicity of this fog only came to light in 1952, when 4,000 people inhaled the fumes, got sick afterwards and died as a result. From that moment on the British government ordered restrictions to be made in the Clean Air Act 1956, so that the amount of factory fog would be drastically reduced. (See also here for more info:http://en.wikipedia.org/wiki/Pea_soup_fog ) It took some time for this to fully take hold; another "Great Smog" came along in 1957 (nowhere near as bad as 1952, but still horrible).
Since then the infamous thick and toxic London fog has disappeared in Real Life (although London's air quality is still rather poor even in the present day) and both in popular culture too, making this a Dead Horse Trope. Still, in older stories, comics, films and novels it can occasionally turn up, as well in tales set in Victorian London, especially Sherlock Holmes and Jack the Ripper and Jack The Rip Off variants, or even during The Edwardian Era, where the fog is pretty much an aesthetic necessity.
See also British Weather.
- A brand of high end trench- and raincoats is named London Fog.
- A brand of British brown ale is also named London Fog. 
- Mobile Fighter G Gundam has an episode set in London, which just happens to fog up during former three-time Gundam Fight champion Gentle Chapman's fights. The fog is actually manufactured to conceal the fact that his wife is sending out dummy mobile suits to help him win, in violation of Article 5 of the Gundam Fight International Regulations that stipulates all Gundam Fight matches to be one-on-one.
- Transformers Headmasters depicts London as a wooded forestland where the locals ride around on horseback. Just to make this even worse, the series is meant to take place in 2011.
- Transformers: Super-God Masterforce has a commercial airline flight coming in to land at a London airport, with the pilots commenting about how thick the fog gets.
- Miyuki from Nurse Angel Ririka SOS thinks that London is like this in her imagine spots.
- Pokémon: One Alola episode features a detective Show Within a Show seemingly set in the Galar region, complete with smog.
- Asterix. In Asterix in Britain, Asterix travels to Great Britain, where they are suddenly (as in, in between saying "the fog falls" and "fast") caught up in fog and can't see anybody or anything. Anticlimax says it is a natural phenomenon in his country.
- Nero. In Het Vredesoffensief van Nero (1951) (The Peace Initiative of Nero) Nero, Petoetje and Madam Pheip walk around in London where the fog makes it impossible to see where they are going or to whom they are talking.
- Blake and Mortimer. Prevalent in The Yellow "M", where Blake and Mortimer walk in the London streets at night and are trying to catch a mysterious criminal.
- Spider-Man. Knight and Fogg were two British super-powered contract killers who appeared in The Spectacular Spider-Man #165-167 back in 1990. The latter saw himself as the personification of the London fog and could transform his body into a gaseous form that obscured his opponents' sight; his favorite method of attack was to strangle his targets from afar with his partially solidified hands.
- Fog is present throughout Zombies Christmas Carol, representing both the plague and encroaching death.
- Blacksad: Blacksad is attacked by a knife-wielding man on a foggy evening, and dryly notes he had the Jack the Ripper impersonation down pat, down to the disappearing in the fog.
- Played for laughs in Nick Knatterton's London adventure Ein Kopf fällt in die Themse ("A head falls into the Thames"), where Nick's investigations are intermittently hampered by the fog. In one scene, he visits a suspect at her home, and when the conversation is finished he leaves. Opening the front door he asks in puzzlement: "Why is there a curtain in front of the door?" — "That's not a curtain, that's the fog."
- Spoofed in a Credits Gag at the end of Cloudy with a Chance of Meatballs. A World-Healing Wave is shown undoing the damage caused by the FLDSMDFR and returning various ruined cities back to their original state. Finally, we see London looking dreary, gray and permeated with fog. The magic wave then passes through, leaving the city still looking dreary, gray and permeated with fog.
- Most movie adaptations of Sherlock Holmes have used foggy London as their setting.
- Pandora's Box ends with Lulu meeting Jack the Ripper on the Ominous Fog-bound streets of London.
- The Lodger, Alfred Hitchcock's first film, is subtitled A Story of the London Fog. A Serial Killer is stalking the foggy streets of London, and the new lodger at Mrs. Bounting's rooming house might be him.
- Gaslight uses this as Ominous Fog to establish mood, like when Paula is being led away from the house after her aunt has been murdered, or later, when her evil husband Gregory is skulking through the streets and alleyways.
- In The Cannonball Run, Seymour Goldfarb (Roger Moore) says this when his car starts filling up with smoke after using the smoke screen and oil slick to get the pursuing police cars off his tail.
- The Case Of The Mukkinese Battle Horn opens with the screen filled with impenetrable London fog. But the narrator cheerfully points out that even in the thickest of fogs, there are some landmarks you just can't miss. Like Nelson's column, for example. *sound of car crashing* "You see? There's someone not missing it now!"
- Lon Chaney vehicle The Blackbird has perpetually fog-bound streets, which help set the atmosphere for London's notoriously sleazy Red Light District of Limehouse.
- In The Ghoul, Laing travels to London and attacks on the street on a foggy night so he can steal her bag and plant a note in it.
- Bleak House. When Esther arrives in London she ask a passer by what this dense brown smoke might be and thinks it is a great fire.
'O, dear no, miss,' he said. 'This is a London particular.' I had never heard of such a thing. 'A fog, miss,' said the young gentleman.
- It is mentioned all throughout A Christmas Carol.
- Near the start of the story, for instance:
Once upon a time—of all the good days in the year, on Christmas Eve—old Scrooge sat busy in his counting-house. It was cold, bleak, biting weather: foggy withal: and he could hear the people in the court outside, go wheezing up and down, beating their hands upon their breasts, and stamping their feet upon the pavement stones to warm them. The city clocks had only just gone three, but it was quite dark already—it had not been light all day—and candles were flaring in the windows of the neighbouring offices, like ruddy smears upon the palpable brown air. The fog came pouring in at every chink and keyhole, and was so dense without, that although the court was of the narrowest, the houses opposite were mere phantoms. To see the dingy cloud come drooping down, obscuring everything, one might have thought that Nature lived hard by, and was brewing on a large scale.
- Later when Scrooge returns to his home at night, where he will be visited by the ghost of Jacob Marley:
The yard was so dark that even Scrooge, who knew its every stone, was fain to grope with his hands. The fog and frost so hung about the black old gateway of the house, that it seemed as if the Genius of the Weather sat in mournful meditation on the threshold.
- And near the end, during the famous moment when Scrooge awakens and feels like a new man and opens the window:
Running to the window, he opened it, and put out his head. No fog, no mist; clear, bright, jovial, stirring, cold; cold, piping for the blood to dance to; Golden sunlight; Heavenly sky; sweet fresh air; merry bells. Oh, glorious! Glorious!
- Near the start of the story, for instance:
- Despite its association with Sherlock Holmes the London fog is only mentioned in a handful of novels, but Ominous Fog of the rest-of-Britain variety nearly ruins Holmes' gambit in The Hound of the Baskervilles and leads to Stapleton drowning in the moor, and is discussed shortly beforehand.
Holmes: [...] and then, Lestrade, we will take the London fog out of your throat by giving you breath of the pure night air of Dartmoor. Never been there? Ah, well, I don't suppose you will forget your first visit.
- A Little Princess: "..the yellow fog hung so thick and heavy in the streets of London that the lamps were lighted"
- In Ture Sventon I London the london fog is so thick that the detective Ture Sventon have to attach a thread with a safety pin to one of the crooks to tail him.
- Lampshaded by one of the detectives in Homicide: A Year on the Killing Streets by David Simon, when they're sent to investigate a murder during a foggy Baltimore morning, "just like Sherlock Holmes".
- The Great Stink in The Difference Engine due to the profusion of Steam Punk industry. Thanks to the politics of the time it gets blamed on Luddite sabotage instead.
- When Nellie Bly arrived in London on the round-the-world trip that she wrote about in Around the World in Seventy-Two Days, "a gray, misty fog hung like a ghostly pall over the city."
- Ankh-Morpork, Discworld's Fantasy Counterpart Culture to London, is known for its "gumbo fog", which is like a pea-souper only thicker, fishier, and with things in it that you'd probably rather not know about.
- In Mr Warrens Profession, Aubrey Warren almost crashes headfirst into an oncoming omnibus due to a combination of high emotion and thick fog.
- In one Sesame Street News Flash, Kermit has gone to London to report on the London Fog. He is interrupted by the London Frog, a Guardsman carrying the London Log, and the London Hog. Then the fog clears up, so they all dance the London Clog.
- Get Smart: London is depicted with fog so thick in the "That Old Gang of Mine" episode that Max and 99 can barely see where they are going, so they ask directions of several people who come walking along. The punchline comes when Max reveals they've been standing in their hotel room the whole time.
- In the Are You Being Served? episode "The Bliss Girl," the fog has entered the building, and into the elevator.
- In one series of episodes of Batman ("The Londinium Larcenies"/"The Foggiest Notion"/"The Bloody Tower"), Batman and Robin travel to Londinium (the Bat-universe's analog to London; actually the Roman name for London) to battle Lord Marmaduke Ffogg and Lady Penelope Peasoup. Not only is Londinium depicted as very foggy much of the time, but Ffogg's weapons are also all fog-based.
- The smog is a plot point in a few episodes of Call the Midwife, particularly the pilot; set during the smog of 1957 (one of the last, and the worst one since the 1952 Great Smog), Nurse Jenny Lee's first patient trips over one of her children's toy fire engines while trying to hang the washing in the garden amidst the pea soup. A concussion and premature delivery result.
- In the short-lived UK crime series Jericho, set in London during The '50s, a PSA at the cinema is warning the public to "breathe through your nose" because of the smog. Later while searching for the murderer of that particular episode, Jericho's car has to be navigated through the smog by a detective standing on the fender, wearing a gas mask and waving a torch.
- The Crown (2016): The Series 1 episode "Act of God" depicts the Great Smog of 1952, which happened only a few months into Elizabeth's reign. Winston Churchill (Prime Minister at the time) initially dismissed it as "just fog" and presented a very limited policy response, to the consternation of the Palace and most everyone else. Churchill was later moved by the sight of one of the Downing Street secretarial pool killed by a lorry that couldn't see her in the pea soup.
- At the climax of the final season of Penny Dreadful, London is blanketed with a toxic fog that reportedly kills thousands of people (although the vampires might have had something to do with it as well) as the first stage in a fortunately averted Apocalypse.
- A newspaper article reporting on a plan to install solar panels in London had to spend a few paragraphs debunking this trope, due to the obvious jokes.
- On a particularly foggy day, a British newspaper reportedly ran this headline: "FOG IN CHANNEL - CONTINENT CUT OFF".
- Shadowrun: London sprawl is commonly referred to as "The Smoke". Thanks to the relaxed environmental standards and the massive factory smoke provided by both London and Birmingham, the fog is back with a vengeance.
- In Fallen London, the weather changes every now and then, but it is very often foggy. Which is impressive, because in this universe, London is underneath the earth. On the rare occasions that the fog lifts, "For a moment it seems like London never fell."
- The trope is namechecked by the "Foggy London" utility, which displays the internal state of the ZX Spectrum game Sherlock while a game is played.
- Bonneton in Super Mario Odyssey is heavily based on England and has very thick smog all over the place. The local Bonneters even speak in a diction faintly evocative of stereotypical British English.
- In The Princess Thieves, After robbing the Marquis of Cheswick, Robin and Oberon loose the watchmen by disappearing into the thick, London fog.
- The Simpsons: In "Treehouse of Horror XV" the third segment is a parody of the Jack the Ripper time period, with Bart and Lisa acting as a Sherlock Holmes and Watson ripoff investigating crimes in Victorian London where the fog is looming everywhere.
- An episode of The Impossibles from the 1960s played with this trope. In "The Terrible Twister" it takes place in foggy London. After he is caught, the Twister is employed to help dispel the thick London Fog, which the Lemony Narrator pokes fun at saying that Londoners can finally see each other.
- An episode of Scooby-Doo and Scrappy-Doo named "The Night Ghoul of Wonderworld" had the gang go to a robotic amusement park designed after Victorian London. As a result this trope is there in all its characters-disappearing foggy glory.