Classical smog (reducing smog) occurs in cool humid climate, it is a mixture of smoke, fog and SO2. It is a reducing mixture/smog. Reducing smog = smoke + fog +SO2 Note Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog has high concentration of oxidising agents and is therefore, called as oxidising smog.