CBSE Class 9 Answered

The meteorological definition of fog is a cloud which has its cloud base on or close to ground, and reduces visibility to less than 1000 metres. 100% humidity at ground level can be reached in different ways.

Our atmosphere is made up of many gases, one of which is water vapour. It can hold a certain amount of water as invisible water vapour at any given temperature. If air is cooled it can hold less water and becomes super saturated. At saturation point, some of the water has to condense to form water droplets, which forms cloud. Fog is made of tiny water droplets suspended in air.

There are several types of fog. 'Radiation fog' is formed on clear, still nights when the ground loses heat by radiation, and cools. The ground in turn cools the nearby air to saturation point, thus forming fog.

'Advection fog' is formed by humid air moving horizontally, being cooled down from below. 'Frontal fog' is formed between warm and cold air in a front. There is also 'sea fog', 'hill fog', 'steam fog' and other forms of fog.