What means smog forms over cities?

Cities are places of concentrated human activity and, if not checked, human can and do produce a lot of air pollutants from chimney in homes and factories and form car exhausts.

These pollutants are Sulfur based gases and small Carbon particles. They can mix with natural fog/mist to form smog because they make the fog denser and dirtier.

Also in good weather the particles and gasses react with sunlight to form a photochemical smog which hangs like a haze over the cities.

Both types of smog are very harmful to human health - THEY KILL PEOPLE - and can be prevented by using catalytic converters on cars and insisting all fuels used in cities are smokeless and that factories scrub their exhaust fumes of pollutants before exhausting them to the air.

This was a lesson learned in Europe during the Industrial revolution. Sadly the developing world does not seem to be learning form these experiences.