(Environment & Ecology)General Studies Paper 3
Question:- 148. Briefly explain the mechanism behind formation of fog. What are the various factors that aid its formation over north-western India in winters? Answer in 250 words.
Dec 23, 2022


Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth’s surface. Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and wind conditions.



Fog forms in a similar way in which clouds are formed i.e. when water vapour condenses. The presence of moisture and a fall in the temperature are key factors for the formation of fog. With the land surface cooling down at night, the air close to the surface also cools down. Since cooler air cannot hold as much moisture as warm air, the water vapour in the air condenses to form fog.

There are several different types of fog formed due to various processes/ mechanisms. They are: radiation fog, advection fog, valley fog, and freezing fog.

  1. 1. Radiation fog/ ground fog forms in the evening when heat absorbed by the Earth’s surface during the day is radiated into the air. As heat is transferred from the ground to the air, water droplets form. It does not reach as high as any of the clouds overhead. It usually forms at night and is said to “burn off” in the morning sun.
  2. 2. Advection fog forms when warm, moist air passes over a cool surface. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. E.g., the Pacific coast of the United States, from Washington to California.
  3. 3. Valley fog forms in mountain valleys, usually during winter. It develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley.
  4. 4. Freezing fog happens when the liquid fog droplets freeze to solid surfaces. Mountaintops that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost. The white landscapes of freezing fog are common in places with cold, moist climates, such as Scandinavia or Antarctica.



North-western India, especially the Indo Gangetic Plain is most vulnerable to fog occurrences, with major, weeks-long spells of dense fog in the months of December and January.

-This fog is linked to four reasons– low winds, low temperatures, availability of moisture and pollution particles which act as surface for condensation.

– Western disturbances, which are storms that originate in the Mediterranean Sea, bring moisture-bearing winds to northwest India. This results in increased moisture levels over the region. In the absence of western disturbances, local moisture sources like water vapour from rivers and soil moisture can also cause fog.

-The fall in temperature along with light winds over the Indo Gangetic Plain results in dense fog over the region.

-Air pollution accentuates the fog as fog condensation usually occurs on particles floating in the air. This fog in turn plays a role in reducing day temperatures because fog blocks sunlight.



Needless to say, fog can have “high spatial variability”, and its intensity can depend on factors like humidity, wind, and temperature.