Theory

• In cities, the occurrence of fog increased along with industrialisation. Records of London weather show that in the early 1700s, there would have been about 20 days of fog every year, but by the end of the 1800s, this had risen to over 50 days.
• 1950s - the average number of particles in city air was greater than in rural areas. The particles acted as condensation nuclei and encouraged fog formation at night, usually under high-pressure conditions.
• In the UK, the Clean Air Acts of the 1950s resulted in a reduction in smoke production and particulate emissions, and a decrease in number of foggy days. Cities undergoing more recent industrialisation events are experiencing more fog e.g. New Delhi and Beijing experience regular winter fogs, resulting in a high death toll when pollutants are trapped, creating a toxic smog