From "An American in Regency England," Louis Simonds writes in March 5, 1810
"It is difficult to form an idea of the kind of winter days in London; the smoke of fossil coals forms an atmosphere, perceivable for many miles, like a great round cloud attached to the earth. In the town itself, where the weather is cloudy and foggy, which is frequently the case in winter, this smoke increases the general dingy hue, and terminates the length of every street with a fixed grey mist, receding as you advance."
Mr. Simond's also writes:
"The inhabitants of London, such as they are seen in the streets, have, as well as the outside of their houses a sort of dingy, smoky look; not dirty absolutely--for you generally perceive clean linen--but the outside garments are of a dull, dark cast, and harmonize with mud and smoke. Prepossessed with a high opinion of English corpulency."
In fact, by the 1800's more than a million London residents were burning coal, and winter fogs became a frequent and pervasive nuisance. How can we tie in these London fogs to Jane Austen's works? Here's an excerpt from an interesting online essay:
Escaping the Fog of Pride and Prejudice
The words of the title of Jane Austen's novel, Pride and Prejudice, shroud the main characters, Elizabeth and Darcy in a fog. The plot of the novel focuses on how Elizabeth and Darcy escape the fog and find each other. Both characters must individually recognize their faults and purge them. At the beginning of the novel, it seems as if the two will never be able to escape the thick fog. The scene at the Netherfield ball makes the marriage of Elizabeth and Darcy much more climactic because the pride and prejudice of both increases greatly during the night....
Elizabeth later declines a proposal from Darcy. He proposed, while his pride and love for Elizabeth were still conflicting. His proposal was like Collins', he felt he was giving Elizabeth a great honor. He told her of his struggle to overcome his dislike of Elizabeth's family. The proposal is so unromantic that Elizabeth returns a harsh rejection. This is when Darcy recognizes his pride and begins to purge it. As a truer character is revealed before Elizabeth, she her own prejudice towards him and quickly loses it. The marriage of Elizabeth and Darcy is such a great one because each had to conquer numerable obstacles to be able to accept the other. The Netherfield ball introduced many of the obstacles which made the marriage seem impossible.