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The Indo-Gangetic plain experiences some of the most persistent fog anywhere in the world. The reasons are partly meteorological and partly geographical.
Monalisa DuttaEach winter, from roughly late December to early February, a dense fog settles across the Indo-Gangetic plain, disrupting aviation, rail and road transport on a scale few other weather phenomena in India achieve.
Having provided weather support for flying operations through several of these seasons, I can attest that fog presents a distinctive forecasting challenge: it forms through a well-understood mechanism, yet the precise timing of its formation and clearance remains among the more difficult short-range predictions in operational meteorology.
The fog of northern India is predominantly radiation fog, which forms through surface cooling overnight. Five conditions are required, and all must be present together.
Cloud absorbs outgoing longwave radiation and re-emits a portion downward, limiting surface cooling. Under clear skies, heat radiates freely to space and the ground cools rapidly.
This condition is the most delicate. Wind of approximately 3 to 7 knots is close to ideal. Calm conditions produce cooling confined to a very shallow layer, generating dew or shallow ground fog rather than a deep layer. Wind above roughly 10 knots mixes the cooled air with warmer air aloft, preventing saturation. Only a narrow band of wind speeds permits gentle mixing sufficient to deepen the cooled layer without destroying it.
The air must contain enough water vapour that cooling brings it to saturation. In practice this means a small dewpoint depression — a narrow gap between temperature and dewpoint. In northern India this moisture is supplied substantially by irrigation across the agricultural plain, and by residual moisture following any passing western disturbance.
Cooling requires time, and the extended nights of late December and January provide it. This is a principal reason the fog season peaks when it does.
As the ground cools, it chills the air in contact with it. When that air reaches its dewpoint, condensation occurs and fog forms.
The mechanism is universal; the severity in this region arises from a combination of local factors.
The Himalayan barrier. The mountains obstruct the northward escape of cold air, causing it to pool across the plain. They also shelter the region from stronger winds which would otherwise disperse the fog.
Extensive irrigation. Agriculture across Punjab, Haryana and Uttar Pradesh maintains a continuous supply of surface moisture through the winter months.
Aerosol loading. Fog droplets require condensation nuclei. The plain carries a high burden of particulate matter from vehicular emissions, industry, domestic fuel and crop residue burning. Abundant nuclei permit fog to form at relative humidity below 100 per cent, and produce a larger number of smaller droplets, which reduces visibility more effectively than fewer larger drops would and makes the fog more persistent.
Flat terrain. The uniform topography of the plain permits fog to extend continuously across hundreds of kilometres, unlike hilly terrain where it remains confined to valleys.
The characteristic diurnal sequence is as follows.
Cooling commences after sunset. Temperature and dewpoint converge through the evening. Fog typically forms between midnight and the early hours, with the densest conditions ordinarily between 0400 and 0800 hours local, around the time of minimum temperature.
Clearance, where it occurs, follows solar heating during mid-morning, generally between 1000 and 1200 hours. Where the layer is deep, clearance may be delayed until afternoon or may not occur at all.
Fog is reported in a METAR as FG where visibility falls below 1,000 metres, and as BR for mist where visibility is 1,000 metres or greater. Where the sky is obscured and no cloud base can be determined, vertical visibility is reported as VV followed by the height in hundreds of feet.
Runway Visual Range, reported separately, becomes the governing parameter for approach and landing in these conditions.
Major aerodromes across northern India, including Delhi, operate CAT III instrument landing systems permitting operations in very low visibility, and a substantial proportion of aircraft and crews are appropriately certified. Operations nonetheless slow considerably, as increased separation is required, and aerodromes without such capability may close entirely.
For flight planning during the season, I would recommend particular attention to alternate selection, as the geographical extent of these events frequently means that the obvious alternates are affected simultaneously. An alternate several hundred kilometres away, or south of the affected region, may be necessary.
Fog forecasting remains genuinely difficult, and the reason is the sensitivity of the outcome to small differences in the governing parameters.
A wind of 4 knots produces dense fog; a wind of 9 knots produces none. A dewpoint depression of half a degree produces fog; two degrees may not. Thin high cloud undetected in the evening may suppress cooling sufficiently to prevent formation entirely. These differences lie at or below the resolution of numerical models, which is why fog forecasts are frequently expressed in probabilistic terms and why they are amended frequently.
Current visibility observations across Indian aerodromes and cities are available on our Live Weather Dashboard.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →