Winter fog over the Indo-Gangetic Plain has long disrupted daily life in North India, grounding flights, delaying trains, slowing road traffic and causing major economic losses every year. While fog itself is a familiar winter feature, scientists have struggled to explain why some fog episodes become unusually dense and linger for days.
A new study by researchers from the Indian Institute of Technology Madras provides a clear explanation. The research shows that rising air pollution is directly responsible for making winter fog thicker, deeper and more persistent across northern India.
The findings are based on 15 years of satellite observations combined with advanced computer simulations. The study, published in the journal Science Advances uses data from NASA’s CALIPSO satellite, which studies clouds and fog using lidar, a laser-based technology that can measure the vertical structure of the atmosphere from space.
Satellite data revealed that when pollution levels are high, fog layers become 15 to 20 per cent thicker compared to fog forming in cleaner air. Polluted fog also contains larger water droplets near its upper layers, making it denser, more opaque and harder to clear. These patterns remained consistent even after accounting for changes in temperature, humidity and wind, confirming that pollution particles play a direct role.
To understand the process in detail, the researchers ran high-resolution weather simulations that included aerosols, the tiny particles released by pollution. They found that these particles act as nuclei on which water vapour condenses, leading to the formation of a much larger number of fog droplets. As droplets form, they release heat, creating gentle upward motion within the fog that helps it grow vertically.
At the same time, fog with a higher concentration of water droplets loses heat more efficiently from its top, especially during the night. This cooling keeps the air saturated, encouraging further condensation and strengthening the fog. Together, these processes create a positive feedback loop that thickens and sustains fog, a phenomenon scientists describe as “fog invigoration.”
The effect is strongest at night, when cool and humid conditions allow pollution particles to activate more efficiently and sunlight is absent to break the fog apart. The study is significant because it extends the concept of pollution-driven invigoration, previously associated mainly with storm clouds, to fog, one of the calmest atmospheric phenomena.
The findings also carry important policy implications. By clearly linking particulate pollution to thicker and longer-lasting fog, the research suggests that improving air quality could reduce not only health risks but also severe winter disruptions affecting millions of people across one of the world’s most densely populated regions.