Formation of Fog

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• Recently, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan.

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• The fog episodes are likely to recur over the Indo Gangetic Plain for the next three days.
• It will keep visibility poor in the hours before and after daybreak.

Fog Formation

• Fog forms like clouds do — when water vapour condenses. 
• Formation:
  • 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.
• Favorable Conditions:
  • The presence of moisture and a fall in the temperature are key factors for the formation of fog. 
  • 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.
  • 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. 

Image Courtesy: media.bom.gov.au

Types of Fog

• Radiation Fog (or ground fog):
  • These fog episodes last for a few mornings on account of calm winds and western disturbances, resulting in localised fog formation.
  • Once the temperature increases during the day, the fog dissipates.
• Advection Fog:
  • Advection fog is larger in scale both in terms of the area covered and duration. 
  • Advection fog forms when warm, moist air passes over a cool surface, causing water vapour to condense. 
  • Advection fog mostly occurs where warm, tropical air meets cooler ocean water. 
  • If the wind blows in the right direction, sea fog can be transported over coastal land areas.
• Valley Fog:
  • It is the result of mountains preventing dense air from escaping.
  • The fog is trapped in the bowl of the valley and can last for several days.
• Freezing Fog:
  • It is the result of liquid droplets freezing on solid surfaces. 
  • Cloud-covered mountaintops often see freezing fog. 
  • These are not applicable to the Indo Gangetic Plain.

Conditions Favouring Fog over Northwestern India

• Decreasing Temperature: 
  • Temperatures have begun to dip over northwestern India. 
  • Recently, Delhi recorded the lowest minimum temperature of the season so far — 6 degrees Celsius. 
  • Cold wave conditions, in which the minimum temperature is significantly lower than normal, have been recorded recently over Punjab, Haryana, and parts of Rajasthan.
• Dense Fog:
  • The fall in temperature along with moisture and light winds over the Indo Gangetic Plain has resulted in dense fog over the region.
• Effect of Western disturbances:
  • 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 vapor from rivers and soil moisture can also cause fog.

Characteristics of the fog over Delhi

• Warmer winter:
  • Delhi saw a warmer start to the winter this year, with maximum temperatures remaining above normal till around mid-December. 
  • Scientists attributed this to a lower number of western disturbances affecting the city. 
  • This means that northwesterly winds did not bring much moisture and did not lead to any significant fog formation until mid-December.
• Highly Variable Fog Episodes:
  • Season to season, it shows very high variability with extreme fog of 25 to 35 days (200 to 285 hours) of dense fog” like in 2017-18. 
  • On the other hand, in some years — like in December 2021 — Delhi hardly sees any dense fog events.
• Radiation fog:
  • An update from the SAFAR forecasting system categorised the recent fog episode in Delhi as “radiation fog”. 

Link between Pollution Levels and Fog

• More Fog at Polluted Places:
  • According to the India Meteorological Department (IMD), Delhi being more polluted, records more fog days compared to others.
• Fog increases Pollution: 
  • 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. 
  • Recently, Delhi recorded a spike in pollution levels with AQI in the ‘severe’ category.
• Rapid Growth of Pollutants: 
  • Advection fog episodes last longer and secondary particulate formation then begins leading to rapid buildup of pollutants. 

Source: IE