Jan. 19 (Bloomberg) -- London’s fog, a fixture in the U.K. capital that led to the deaths of 4,000 people in 1952, may be on the wane, contributing to warmer temperatures. Why? Cleaner air.

French-led scientists studied three decades of data from 342 weather stations across Europe and found that on average, the number of days where visibility was lower than 2 kilometers (1.2 miles) has halved since 1980. The trend correlates with a fall in emissions of sulfur dioxide, a gas associated with burning coal and oil, they said yesterday in the Nature Geoscience journal.

“We’ve moved away from using coal and wood in the home and our industries have become much cleaner,” Dave Britton, a meteorologist at the U.K. government forecaster, the Met Office, said in a telephone interview from Exeter. “So we see much less of that particulate matter which allows water to condense.”

Fog and mist form when water droplets are suspended around particles in the air. The decline in fog across Europe may also have contributed to warming of the continent in recent years, said the study’s authors, led by Robert Vautard, a scientist at the French Atomic Energy Commission outside Paris.

“By enabling less energy to be received at the surface during daytime, the low-visibility phenomenon inhibits surface heating and therefore induces a lower local temperature,” the researchers wrote.

The decline in fog has raised temperatures by 0.08 degrees Celsius (0.14 Fahrenheit) per decade across Europe, or up to a fifth of the total warming observed, the scientists calculated. In eastern Europe, the drop in fog may account for half of the total warming, they said. The influence on warming of declining fog will wane in the future because there are fewer fog days to lose as the air becomes cleaner, they said.

Rising U.K. Temperatures

The Met Office’s Central England Temperature series, which includes London, shows that the average for the 30 years ending in 2008 was 9.95 degrees Celsius, up almost a half-degree from the 9.49-degree average for the 30 years ending in 1978, Britton said.

Global warming may also have contributed to the drop in fog because cooler days favor its formation, said Britton, who wasn’t involved in the study. That, coupled with the study’s finding that the drop in fog adds to warming, would make a so-called “positive feedback” whereby warmer temperatures trigger events that themselves warm the climate further, he said.

The biggest declines in days with visibility of 5 kilometers or less have taken place in northern and eastern European nations, including the U.K., Germany, Poland, the Netherlands and former members of the Soviet Union, according to the paper.

Famous Fog

London’s famous fogs date back at least 700 years when King Edward I banned the burning of a highly polluting form of coal to improve the city’s air. In the 1800s, industrialization led to fog that lingered for weeks. The so-called “Great Smog” of 1952 led to the deaths from heart and lung disease of about 4,000 people, according to the Met Office.

While fog is on the wane, there are some times of year when you’re more likely to see it in the U.K., according to Britton. One such time is around Nov. 5, he said. Around that day, known as Guy Fawkes Night, Britons set off fireworks and burn effigies on bonfires to commemorate the 1605 Gunpowder Plot to blow up Parliament, led by a man called Guy Fawkes.

“Anecdotally, every Nov. 6 is usually much foggier because it’s the day after bonfire night,” Britton said. The Met Office doesn’t have statistics to back that up, he added.

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: January 19, 2009 11:38 EST