Fog

On May 19, 1780, a remarkable event known as the "Dark Day" occurred in Eastern Canada and the New England area of the United States. The day began as usual, but around 10:30 AM, a thick darkness suddenly enveloped the region. The sky turned a deep, almost black color, and it was so dark that candles were needed to see indoors. Many people described it as if night had fallen during the daytime.

The cause of this unusual phenomenon was a combination of factors, including forest fires and a dense fog. The smoke from extensive forest fires, likely in the region of what is now Ontario, Canada, along with the fog and heavy cloud cover, created the conditions for the Dark Day. The smoke particles and moisture in the air scattered and absorbed sunlight, resulting in the profound darkness that lasted for several hours.

The Dark Day had a significant impact on the people of New England. Many people were afraid that the darkness was a sign of the end of the world. Some people even went to church to pray for deliverance. The darkness also disrupted travel and commerce. Ships were unable to sail, and businesses were forced to close.

When heavy fog seeped into Western Pennsylvania in 1948, mixed with industrial pollutants, the air became poisonous and killed 20 people in the area.

Practically the whole of the London area was paralysed on the night of December 12-13 1946 in one of the worst fogs in memory. At one stage there was a traffic jam extending for nearly a mile from Greek Park across Hyde Park corner to Knightsbridge and most of the 4,000 people who attended the ice hockey matches at Wembley had to stay there the whole night. At 2 a.m. thousands of cars, buses, trams and taxis had been abandoned in streets.

The "Great Smog" began in London on December 5, 1952 and lasted for five days, causing  10,000- 12,000 deaths and making 150.000 people ill. It led to the Clean Air Act 1956.

Nelson's Column during the Great Smog of 1952. By N T Stobbs, Wikipedia

Some of the approx. 11,000 people who died in the five days the Great Smog of London lasted didn't suffer lung problems – they fell into the Thames and drowned because they could not see the river.

The term 'pea souper coined' by Londoners for smog, refers to the greenish yellow hue of fog that contains both soot and poisonous sulphur dioxide.

During a football match between Chelsea and Charlton Athletic, the fog was so thick that Charlton keeper Sam Bartram was unaware that the game has been stopped by the referee due to poor visibility, assuming play was in Chelsea's half. It was 15 minutes before the police told him he could stop guarding the goal as the game had been abandoned.

The 1952 Great Smog of London was so bad that the fog seeped indoors and movie screenings and concerts had to be cancelled because it was more difficult to see inside a large room like a movie theater. 

For aviation purposes 3,280 feet is the visibility distance below which mist is defined as fog.