New international research led by Indian Institute of Technology Madras (IIT Madras) has provided compelling new evidence that heavy particulate pollution is a key driver behind the increasing frequency and persistence of dense winter fog over North India.

New findings published in Science Advances (https://doi.org/10.1162/sciadv.adx1662) (by the American Association for the Advancement of Science) and Geophysical Research Letters (https://doi.org/10.1029/2024GL114095) (by the American Geophysical Union) reveal that high amounts of particulate pollution at nighttime are the primary reason for the intensification of fog.

How Pollution Causes Fog: A Microphone Effect

The study led by N. Arun and Prof. Chandan Sarangi of the Civil Engineering Department, IIT Madras, in collaboration with international experts, explains that a cocktail of nighttime particles favors the intensification and duration of fog events over North India.

The calm winter nights over the region favor the accumulation of pollutants near the ground. As the air temperature drops, water vapor quickly condenses on these particles, resulting in the formation of water droplets that are typically a few micrometers in size. This large number of droplets (typically, for fog, between 1 and 50 million droplets per cubic meter) obstructs light and reduces visibility.

As these droplets form, they release “latent heat” (hidden energy). This heat causes the fog to grow vertically, reaching nearly 600-800 meters-much higher than previously expected. The next morning, this thick layer acts as a massive shield, blocking the sun. Because solar energy does not reach the surface, natural heating of the ground is reduced, slowing down the evaporation of fog from the surface, which leads to “dull sky” conditions that ground flights and delay trains.

Mr. N. Arun, doctoral scholar, Department of Civil Engineering, IIT Madras), said, “If the fog layer persists, then the solar heating of the surface is further dampened, leading to more favorable conditions for condensation to occur the next night. Previously, scientists believed that the root cause behind the increasing fog frequency to wind circulation changes over North India associated with global warming; however, we are now understanding that the regional pollution can also play an equally important role in prolonging fog events.”

“These findings provide a better understanding of how pollution can impact fog intensity and duration. This understanding can be critical for improving fog forecasting can help pilots and airports to provide better and informed decisions”, said, Dr. Chandan Sarangi Associate Professor at Civil Engineering, IIT Madras.

The research offers a glimmer of hope: fog is closely tied to particulate pollution levels over North India. Improving air quality can directly reduce the frequency and severity of fog events during winter days.

The Research was undertaken by Lead Authors N Arun and Prof. Chandan Sarangi from IIT Madras, with co-authors Dr. Vijay P. Kanawade from Cyprus Institute, Cyprus, Dr. Ritesh Gautam from Environmental Defence Fund, US, Dr. Manoj K Singh from Bennett University, India and Dr. Yun Qian from Pacific Northwest National Laboratory, US.