During winter mornings, sometimes we see a smoky cover on the Earth’s surface. This is called fog. When fog is very dense, the visibility becomes poor, and it becomes difficult to see anything. Fog is a kind of cloud, which comes in contact with the ground. It is formed by the condensation of water vapour present in the air close to the ground. We know that water vapour is always present in the air. Air cannot hold more than a certain amount of water vapour at a given temperature. As the temperature decreases, the surplus water vapour converted into small particles of water or ice. When such condensation takes place near the Earth’s surface, a cloud or fog is formed. The minute particles of water present in the fog reduce visibility to a great extent. During winters, the Earth’s surface is cool in the morning, so the water vapour present in the air condenses into fog. The layers of fog are thicker in big cities than in villages and smaller towns. This is so because there is a greater degree of dust particles and smoke in big cities. Dust particles and smoke get mixed with water particles present in the fog and make it denser. This type of fog is also called smog. You must have noticed that fog is denser in those cities where there is more smoke due to the large number of factories. In big industrial cities like Mumbai, Kolkata and Delhi, this type of dense fog is a common feature. The presence of fog causes great inconvenience to transport systems. Sometimes, planes and motorcars meet with accidents because of the poor visibility. In 1955, a chemical was developed to clear the fog. Silver iodide or calcium chloride is sprayed in foggy areas, which enables water particles present in the fog to fall in the form of raindrops. However, there is no method, which has been successful in clearing fog till date. Smog can cause throat problems.