How Air Pollution Thickens Winter Fog in the Indo-Gangetic Plain

 

How Air Pollution Thickens Winter Fog in the Indo-Gangetic Plain

(For UPSC GS Paper I & III – Geography, Environment, Science & Tech, Current Affairs)

Winter fog is a recurring hazard across the Indo-Gangetic Plain (IGP), disrupting transport, agriculture, aviation, and daily life. A new study by IIT Madras, published in Science Advances (January 9), provides crucial scientific insight into why fog in this region becomes so persistent and dense.

The research shows that aerosols (pollutants like dust, smoke, soot) present above the fog layer make the fog thicker and longer-lasting.

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Key Scientific Findings

1. Vertical Structure of Fog

Fog thickness depends on:

• Base: Remains near the ground.

• Top: Rises upward when pollution is high.

Using 15 years of CALIPSO satellite data, researchers found:

• On polluted days, the top of the fog layer rises, increasing overall thickness by about 17%.

• The base stays fixed, meaning fog becomes vertically deeper.

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2. Role of Aerosols (AODFOG Index)

Researchers developed a new index called AODFOG to measure:

• The amount of aerosols above the fog layer.

Comparisons showed:

• High AODFOG days (more pollution above fog) → thicker fog.

• Low AODFOG days → thinner fog, clears faster.

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3. Droplet Growth (MODIS Satellite Data)

Using MODIS satellite observations, scientists found:

• Fog droplets near the top were larger on polluted days.

• Larger droplets mean more condensation and stronger fog persistence.

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The “Vicious Cycle” Mechanism

A weather model simulation of a major January 2014 fog event revealed a self-reinforcing loop:

1. More aerosols = more condensation nuclei
   → More “seeds” for water vapour to condense.

2. More condensation releases latent heat
   → This heat helps stir and mix the fog upward.

3. Thicker fog emits infrared radiation efficiently
   → Cools the upper fog layer, keeping it humid and favourable for more condensation.

4. Result:
   A positive feedback loop:

   Pollution → Thicker fog → Longer persistence → More trapped pollution

This explains why winter smog-fog episodes in North India last for days.

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Additional Insight: Semi-Direct Effect of Soot

Soot can:

• Absorb sunlight,

• Warm air near or above fog,

• Alter cloud and fog dynamics.

However, this effect could not be fully studied due to:

• Poor data on aerosol properties above fog,

• Sparse vertical observations (a key limitation of the study).

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UPSC Relevance

GS Paper I – Geography

• Atmospheric stability and temperature inversion in winter

• Fog formation over alluvial plains

• Role of aerosols in microclimate modification

GS Paper III – Environment & Science

• Air pollution–weather interaction

• Aerosol–cloud–radiation interaction

• Remote sensing (CALIPSO, MODIS satellites)

• Climate feedback mechanisms

GS Paper II – Governance

• Aviation safety, disaster preparedness

• Urban air quality management

• Interlinkage between environment and economic productivity

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Answer Enrichment Quotes

“North India’s winter haze is a vicious cycle: aerosols fuel fog, fog traps pollution.”

— Chandan Sarangi, IIT Madras

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Way Forward (Policy-Oriented Points)

1. Pollution Control = Fog Mitigation

   • Reducing PM2.5 and black carbon can shorten fog duration.

2. Improved Vertical Monitoring

   • More LIDAR and satellite-based profiling of aerosols.

3. Early Warning Systems

   • Integrating aerosol data into fog forecasting models.

4. Co-benefits

   • Better air quality → Fewer flight cancellations → Economic gains → Health improvement.

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Conclusion

The IIT Madras study scientifically proves that air pollution does not just coexist with fog — it actively strengthens it. Winter fog in the Indo-Gangetic Plain is not merely a meteorological phenomenon but an aerosol-driven, self-amplifying environmental hazard, making air quality management a crucial tool for climate resilience and public safety.