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Formation of Fog

Context:

For two consecutive mornings, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan.

Relevance:

GS I: Geography

Dimensions of the Article:

  1. How does fog form?
  2. What’s been happening over northwestern India?
  3. What is the link between pollution levels and fog?

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.

What’s been happening over northwestern India?

  • Temperatures have begun to dip over northwestern India.
  • Cold wave conditions, in which the minimum temperature is significantly lower than normal, have been recorded recently over Punjab, Haryana, and parts of Rajasthan.
  • The fall in temperature along with moisture and light winds over the Indo Gangetic Plain has resulted in dense fog over the region, according to the India Meteorological Department (IMD).
  • 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  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.

What is the link between pollution levels and fog?

  • Delhi being more polluted, records more fog days compared to others.
  • Delhi recorded a spike in pollution levels with AQI in the ‘severe’ category.
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. This is the radiation fog that we are seeing in Delhi.
Advection fog:
  • This happens 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.

-Source: Indian Express


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