A new study by researchers at the Indian Institute of Technology Madras (IIT-M) has revealed a concerning link between aerosols and the increasing severity of winter fog in North India. The research, published in the journal Science Advances on January 9, 2026, indicates that aerosols aloft are contributing to the thickening of winter fog layers, exacerbating air quality issues and disrupting weather patterns across the Indo-Gangetic Plain.
Winter fog is a common phenomenon in North India, often leading to reduced visibility and significant disruptions in daily life. The IIT-M study highlights that this fog is frequently intensified by pollution, forming in layers of polluted air close to the ground and prolonging fog events. Understanding the vertical structure of this fog, particularly its thickness, is crucial for predicting its duration and potential impacts.
The researchers analyzed 15 years of data from the CALIPSO satellite to investigate the impact of aerosol loading above fog layers. They introduced a new metric called AODFOG to quantify the amount of dust and smoke present in the atmosphere above the fog. The findings indicate a direct correlation between aerosol levels and fog thickness: fog layers are approximately 17% thicker on days with high aerosol loading compared to days with low AODFOG. The top of the fog layer rises higher with elevated pollution levels, while the base remains near the ground, and water droplets near the top of the fog layer are larger on days with high AODFOG.
To further understand the dynamics of fog formation and persistence, the researchers used MODIS satellite data to estimate the size of water droplets within the fog and a weather model to simulate a significant fog event from January 2014. The model suggested a self-strengthening cycle where increased aerosol loading enhances fog droplet number concentrations, leading to greater latent heat release. This, in turn, results in buoyancy-induced vertical mixing within the fog. Simultaneously, the increased water content intensifies longwave radiative cooling at the fog top, promoting cool-saturated conditions that further favor the formation of condensate and larger droplets. The team also noted that soot can absorb sunlight and warm air near or above the fog, though they didn't isolate this effect due to poorly known aerosol properties and sparse observations. The aerosol-induced vertical mixing, driven by thermodynamic uplift and radiative cooling, amplifies condensational processes, causing fog invigoration, particularly at nighttime.
The study highlights a troubling feedback loop: aerosols contribute to fog formation, while the fog traps pollution, leading to deteriorating air quality. This cycle not only affects visibility but also has broader implications for public health and daily activities. Chandan Sarangi, an earth system scientist at IIT-Madras and the corresponding author of the study, emphasized the importance of addressing air pollution, stating that "Tackling air pollution can clear skies, boost health, and energize the economy".
These findings underscore the complex interactions within the atmosphere and the significant role of aerosols in influencing weather patterns and air quality. As North India continues to grapple with persistent fog and pollution, a deeper understanding of these dynamics is essential for developing effective mitigation strategies and protecting public health.
