Smog is a combination of the words smoke and fog. We can distinguish two separate types of smog, summer smog and winter smog. Summer smog, mainly consists of ozone. It is a brown, oxidizing fog. The causes of photochemical smog are nitrogen oxides and VOC, which stem from traffic and industries. Winter smog is also referred to as acid smog; it mainly consists of foggy elements. Winter smog is found in areas where vertical dispersion of air pollutants is not possible. Usually temperatures decrease during the day in high air layers. The reason for poor visibility in Northern India in winter is not only smoke. It is mist pollution and smoke that people mistake for fog.