Pollution and ozone combined creates smog
It comes from heavy motor vehicle, high temperatures, sunshine, and calm winds
When the temperatures does the opposite the warm air stays near the ground instead of rising also the winds are calm
Smog could stay in your city for days
As traffic and other origins add more pollutants to the air the smog gets worse
Smog exist in major cities like California, SAN Francisco, San Diego, China, etc.