IAS Gyan

Daily News Analysis


21st December, 2022 Geography

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How does fog form?

  • Fog forms like clouds do — 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.
  • Fog begins to form in the early hours of the morning, when the temperature is at its lowest.
  • Fog can have “high spatial variability”, and its intensity can depend on factors like humidity, wind, and temperature.
  • Areas near water bodies, for instance, may see denser fog because of the higher humidity.

Fog in Indo Gangetic region

  • Moisture is available in the atmosphere and winds are light at lower tropospheric levels over Indo-Gangetic plain. Due to clear sky and overnight cooling, the surface of earth down rapidly and air near the Earth's surface also become colder and heavier. The moisture available in the air condense and overcome fog. 
  • Western disturbances, which are storms that originate in the Mediterranean Sea, bring moisture-bearing winds to northwest India. This can result 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.
  • According to a note issued by the IMD, 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. These foggy spells are linked to wind and temperature patterns.


Link between pollution levels and fog

Radiation Fog

  • As temperature declines, local wind speed also falls. The inversion layer comes down and vertical mixing reduces. This results in fog formation and particulate matter hangs on the boundary layer, increasing pollution levels. Once the temperature increases during the day, the fog dissipates. 


Advection Fog

  • Advection fog, occurs when the humidity is much higher. These fog episodes last longer and secondary particulate formation then begins leading to rapid build-up of pollutants. Lower temperatures across the Indo Gangetic Plain in January can cause such fog episodes.