During winter mornings, sometimes we see a smoky cover on the Earth’s surface. This is called fog. When becomes very dense, the visibility is 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 near the ground. We know that water vapours are always present in air. Air cannot hold more than a certain amount of water vapour at a given temperature. The water vapours in excess of that limit are converted into small particles of water or ice. When such condensation takes place near the Earth’s surface, a cloud of fog is formed. The minute particles of water present in the fog reduce visibility to a great extent. During winter, the Earth’s surface is cool in the mornings, as such, water vapour present in the air condense 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 fogs are denser in those cities where there is more of smoke due to the large number of factories. In big industrial cities like Bombay, Calcutta and Delhi, this type of dense fog is a common feature.
The presence of fog causes a great inconvenience to the transport system. Sometimes, planes and motor cars meet with accidents because of the poor visibility due to fog. In 1955, a chemical method was developed to clear the fog. In this method, silver iodide or calcium chloride is sprayed in foggy areas which enable water particles present in the fog to fall down on the Earth in the form of raindrops. However, no system has succeeded fully in clean ring fog so far.
