For two consecutive mornings, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan. The fog episodes, which follow a comparatively warm start to the winter, are likely to recur over the Indo Gangetic Plain for the next three days, keeping visibility poor in the hours before and after daybreak.
Fog forms like clouds do — when water vapour condenses. The presence of moisture and a fall in the temperature are key factors for the formation of fog. With the land surface cooling down at night, the air close to the surface also cools down. Since cooler air cannot hold as much moisture as warm air, the water vapour in the air condenses to form fog.
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Fog begins to form in the early hours of the morning, when the temperature is at its lowest. On Monday, for instance, fog in Delhi began to form around 1.30 am.
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Fog can have “high spatial variability”, and its intensity can depend on factors like humidity, wind, and temperature, R K Jenamani, scientist, IMD, said. Areas near water bodies, for instance, may see denser fog because of the higher humidity.
What’s been happening over northwestern India?
Temperatures have begun to dip over northwestern India. On December 17, Delhi recorded the lowest minimum temperature of the season so far — 6 degrees Celsius. Cold wave conditions, in which the minimum temperature is significantly lower than normal, have been recorded recently over Punjab, Haryana, and parts of Rajasthan.
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The fall in temperature along with moisture and light winds over the Indo Gangetic Plain has resulted in dense fog over the region, according to the India Meteorological Department (IMD).
Western disturbances, which are storms that originate in the Mediterranean Sea, bring moisture-bearing winds to northwest India. This can result in increased moisture levels over the region. In the absence of western disturbances, local moisture sources like water vapour from rivers and soil moisture can also cause fog, according to Jenamani.
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According to a note issued by the IMD, the Indo Gangetic Plain is most vulnerable to fog occurrences, with major, weeks-long spells of dense fog in the months of December and January. These foggy spells are linked to wind and temperature patterns.
What are the characteristics of the fog over Delhi?
Delhi saw a warmer start to the winter this year, with maximum temperatures remaining above normal till around mid-December. Scientists attributed this to a lower number of western disturbances affecting the city. This means that northwesterly winds did not bring much moisture and did not lead to any significant fog formation until December 19.
An update from the SAFAR forecasting system on Monday categorised the fog episode in Delhi on Monday as “radiation fog”. Radiation fog (or ground fog) episodes last for a few mornings on account of calm winds and western disturbances, resulting in localised fog formation.
In contrast, “advection fog” is larger in scale both in terms of the area covered and duration. Advection fog forms when warm, moist air passes over a cool surface, causing water vapour to condense. Advection fog mostly occurs where warm, tropical air meets cooler ocean water. If the wind blows in the right direction, sea fog can be transported over coastal land areas.
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Other kinds of fog include “valley fog” — which is the result of mountains preventing dense air from escaping, and in which the fog is trapped in the bowl of the valley and can last for several days — and “freezing fog”, which is the result of liquid droplets freezing on solid surfaces. Cloud-covered mountaintops often see freezing fog. These are not applicable to the Indo Gangetic Plain.
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Fog episodes over Delhi are highly variable, according to the IMD. “Season to season, it shows very high variability with extreme fog of 25 to 35 days (200 to 285 hours) of dense fog” like in 2017-18, the IMD says. On the other hand, in some years — like in December 2021 — Delhi hardly sees any dense fog events.
The highest number of dense fog (when visibility is less than 200 m) and very dense fog (visibility less than 50 m) days are usually seen in January in Delhi. Going by a 31-year average till 2021, Delhi records around 25.3 hours of “very dense” fog in December, and 38.3 such hours in January.
What is the link between pollution levels and fog?
The note issued by the IMD on fog episodes mentions that “Delhi being more polluted, records more fog days compared to others”.
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On Monday, Delhi recorded a spike in pollution levels with AQI in the ‘severe’ category. On this, Dr Gufran Beig, founder project director, SAFAR, said, “As temperature declines, local wind speed also falls. The inversion layer comes down and vertical mixing reduces. This results in fog formation and particulate matter hangs on the boundary layer, increasing pollution levels. Once the temperature increases during the day, the fog dissipates. This is the radiation fog that we are seeing in Delhi.”
Pollution levels can also impact fog in a situation that Delhi is yet to witness this year. “The second situation which has not yet arrived, but which we are likely to see in the last week of December and in January, is advection fog, when the humidity is much higher. These fog episodes last longer and secondary particulate formation then begins leading to rapid build up of pollutants. Lower temperatures across the Indo Gangetic Plain in January can cause such fog episodes. Winter has only just arrived in Delhi,” Dr Beig said.
