9. Fog

Fog is a special type of thin cloud consisting of very small water droplets which remain suspended in air close to the surface of the Earth. Fog is formed due to condensation of water droplets suspended in the atmosphere in the vicinity of the earth’s surface under certain conditions, such as low temperature and high relative humidity.. During the winter season, excessive radiation at night results in the fall of air temperature. The condensation of water vapour takes place around the dust and smoke particles that remain suspended in the air. It is called fog. The formation of fog near the surface of the Earth does not involve ascent and consequent expansion of air. The visibility is greatly reduced (less than one km).

Fog is of three types:

1) Radiation Fog: The surface is cooled at night due to terrestrialradiation and the air which come into contact with it also gets cooled. Consequently tiny droplets forming the clouds are called radiation fog. It is not very thick and this thickness varies from 10 to 30 metres.

2) Advection Fog: It is formed when there is fall in temperature of warm moist air moving horizontally over a cold surface. It is cooled by contact and sometimes by mixing with cold air prevailing over cold surfaces.

3) Frontal or Precipitation Fog: The dividing line separating cold and warm air masses are known as fronts. At these fronts convergence of warm and cold air takes place and fog is formed. The warm air in the frontal area is light and rises above the cold air mass. It then begins to cool and when the temperature reaches dew point, frontal fog is formed.