London fog — the famous "pea souper" caused by coal smoke mixing with natural Thames valley mist — was one of the most distinctive visual phenomena of 19th century European culture. Monet made 100 paintings in London specifically to capture the fog, writing that "without the fog, London wouldn't be beautiful." The fog transformed ugly industrial forms into atmospheric mystery.

Softening, obscuring, and slightly melancholic. Fog reduces the world to its essentials — forms lose detail, distances become uncertain, familiar landscapes become mysterious. Psychologically it creates both unease (loss of visual information) and beauty (the softening of harsh edges). Associated with ambiguity, potential, and the suspension of certainty.

Monet's 100 London fog paintings — made during extended stays in 1899–1904 — represent one of the most intensive explorations of atmospheric colour in art history. He wrote to his wife that he could only work on each canvas for seven minutes before the light changed, eventually working on 80–100 canvases simultaneously. The London fog of Victorian and Edwardian times, while visually beautiful, was a public health catastrophe — the Great Smog of 1952 killed approximately 12,000 people and directly led to the Clean Air Act 1956.