London fogs are scientific fact. These notorious pea soup fogs shrouded the actions of Jack the Ripper during his reign of terror on the Victorian streets, and made it difficult for German bombers to follow the Thames to bomb London during World War II. They are so famous that it is unthinkable to make a Sherlock Holmes movie which does not include the legendary fogs of London.
What causes London fogs?
The capital of the United Kingdom has always been prone to fog and mist because of the tidal Thames River, which opens to the ocean. Many mornings have at least some fog or mist by the river. Further away from the urban center and the river, there is much less fog, although a foggy morning can be counted on about 5 to 10 times a year. Mist is more frequent, but is not as thick and does not stay around as long.
The only difference between fog and mist is how far you can see in it. In the UK, it is considered fog if you cannot see more than 1,094 yards away. For driving purposes, the visibility limit is 319 yards.
Standard nighttime radiation fog happens when warm, moist air over the city cools down and the water vapor precipitates out into cloud form. The urban island effect causes the center of London to be as much as 9 degrees warmer than the outskirts. London's location by the Thames ensures that this warm air is completely saturated before it cools down for the night.
If the surface temperature drops below freezing, London fog can turn into a freezing fog. In this case, the water droplets suspended in the fog precipitate into a slick, icy covering on all surfaces it touches. Frost can be expected twice a week in the depths of winter, but freezing fog won't accompany most frosts.
Most London fogs happen in winter, when the air can't hold as much water vapor. The fogs are densest near the docks, especially east near Tilbury and west towards Heathrow. In case of heavy fog, the Met Office will issue a yellow warning, or an amber warning for fog which disrupts travel for several days. London fog is not considered to have high impact on activities, a designation reserved for extreme storms, so there is no red warning for fog.
These fogs are not as dense as they used to be before the Clean Air Act was passed in 1956. Before that time, the natural fog was made much worse by the smoke from thousands of domestic coal fires. These pea soup fogs were usually blackish, yellow, or yellow-green, rather than the grey-white of clean fog. They were also dry and smoky, rather than the damp mist which is usually associated with fog.
What was pea soup London fog?
Before the Clean Air Act was passed, any windless temperature inversion would trap coal smoke in the atmosphere. To make matters worse, the sulfur dioxide in the smoke combined with water vapor to produce sulfuric acid. This resulted in thick, caustic hazes which could last for days at a time.
The Great Smog of December 1952 was among the worst events of this kind ever documented. It took place during winter, which meant that even more people were burning coal to stay warm than usual. When the temperature inversion came, all that coal smoke was caught at ground level. During the first night, this combined with normal nighttime radiation fog from the cooling moist air caught near the ground.
Visibility closed down to just a few yards or even less, so that all public transportation except the Underground had to shut down. People were not able to see their own feet. The fog also seeped into indoor spaces, so that theater and concert stages could not be seen from the audience and were mostly closed down.
The Great Smog lasted for 5 days during winter, which meant that even more people were burning coal to stay warm than usual. At the end of that time, at least 4,000 and possibly as many as 12,000 people had died from the poor air, with another 100,000 sick. The Clean Air Act was passed just 3-1/2 years later.