North India woke up to a foggy morning in parts of western Uttar Pradesh in early March, just as the weather was supposed to be warming up, and cities like Ghaziabad and Meerut were covered in it. Visibility dropped fast, and the usual morning rush slowed way down, especially on big roads like the Delhi–Meerut Expressway. Drivers found themselves crawling along, headlights barely cutting through the grey haze that settled over highways and neighborhoods.

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Why Did Dense Fog Suddenly Appear During a Warm March Morning?

• Lingering Winter Atmospheric Conditions: Although the winter season is ending, cold northerly winds are still prevalent, creating winter atmospheric conditions, which allow the formation of fog.
• High Humidity Levels: Since the formation of fog depends on high humidity, calm winds, and low visibility, the calm winds allow the formation of thick fog, as the wind speed is low.
• Temperature Inversion Effect: During temperature inversions, a thick layer of warm air forms over cold air, trapping moisture close to the earth’s surface, which allows the formation of thick fog.
• Sudden Collapse of Visibility: In some places between Meerut and Ghaziabad, visibility was reported to be zero, forcing drivers to reduce their speed.

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How Does Pollution Contribute to the Thickness and Density of the Fog?

• Aerosol Particles: The pollution generated by vehicles, industries, and construction activities provides surfaces with aerosol particles that cause the condensation of the water vapour into droplets, which eventually form the thick fog.
• Fog is 15-20% Thicker in Polluted Conditions: Studies using satellite images and data from NASA’s CALIPSO have found that pollution can cause the thickness of the fog to rise by as much as 20%.
• Extreme Pollution Levels in the NCR Region: Cities such as Ghaziabad have seen pollution levels as high as 224 µg/m³, which is almost four times the safe limit set by the WHO, which is merely 25 µg/m³.
• Local Factors: Traffic, construction activities, and industrial pollution, along with the stagnation of the winds, have now been identified as the main reasons that cause pollution and the resultant haze-fog in the Indo-Gangetic plains.

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Could This Be “Flash Fog” and What Does It Mean for the Future?

According to meteorologists, this occurrence is similar to a phenomenon called flash fog, also known as rapid-onset fog. Unlike traditional fog, which develops gradually overnight, flash fog can develop suddenly in response to a combination of moisture, temperature, and calm winds. In this case, visibility can drop significantly in a very short period, only to disappear just as quickly once the sun’s warmth reaches the ground and increases air circulation.

Thus, with the onset of climate change, the seasons being disrupted, and pollution levels increasing in cities, the occurrence of the phenomenon whereby North India woke up to a foggy morning in the middle of March could become much more common in the future.

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