Winter fog is a common problem in the Indo-Gangetic Plain. It reduces visibility for hours, impacting daily life. The fog forms in polluted air near the ground and tends to last longer on polluted days. Scientists from IIT-Madras studied 15 years of satellite data from CALIPSO to understand how pollution affects fog thickness. They found that aerosols like dust and smoke above the fog make the fog layer about 17% thicker by raising its top while the base stays near the ground. Water droplets near the fog top were also larger on polluted days, based on MODIS satellite data. The team created a new number, AODFOG, to measure how much pollution sits above the fog. They compared fog layers on low and high pollution days in a region prone to thick fog. Using a weather model to study a January 2014 fog event, they discovered a cycle where more pollution creates more droplets. These droplets release heat as vapor condenses, stirring the fog upwards and helping it grow thicker. The fog also loses heat efficiently, keeping air cold and humid near the top, allowing more fog to form. Dr. Chandan Sarangi, IIT-Madras earth system scientist and study lead, told The Hindu, "North India’s winter haze is a vicious cycle: aerosols fuel fog, fog traps pollution, impacting air quality, aviation, and daily life. Tackling air pollution can clear skies, boost health, and energise the economy." The researchers also noted that soot can absorb sunlight and warm air near or above the fog, a 'semi-direct' effect not fully studied due to limited data. This study was published in Science Advances on January 9, 2026.