Smog is often caused by heavy traffic, high temperatures, sunshine and calm winds. These are few of the factors behind increasing level of air pollution in atmosphere. During the winter months when the wind speeds are low, it helps the smoke and fog to become stagnate at a place forming smog and increasing pollution levels near the ground closer to where people are respiring. It hampers visibility and disturbs the environment. - See more at: http://www.conserve-energy-future.com/SmogPollution.php#sthash.6t90QK4f.dpuf