Every winter, fog becomes an invisible disruptor of India’s transport systems. Airlines and the Indian Railways face repeated delays, cancellations, and network congestion, particularly across northern India. Despite advances in technology and operational planning, fog remains a hard constraint where safety trumps speed. The challenge is not merely meteorological, but institutional — how to move millions safely when visibility collapses for days at a time.
Why fog in north India is unusually persistent
The most disruptive fog in northern India is radiation fog. It forms on clear winter nights when the ground cools rapidly, moisture condenses near the surface, and stagnant winds trap the fog close to the ground. Unlike hill or advection fog, radiation fog can recur night after night and reduce visibility to just a few metres.
Air pollution worsens the problem. Smog particles make fog denser and prevent it from dispersing quickly after sunrise. This is why cities such as Delhi experience prolonged low-visibility conditions, sometimes lasting through the day, amplifying disruption across transport networks.
How aviation manages low visibility
When visibility drops sharply, airports activate Low Visibility Procedures (LVP). These involve specialised systems such as Low Visibility Take-Offs and precision landings using the Instrument Landing System (ILS).
The highest category commonly used in India is ILS CAT IIIB, which allows aircraft to land with visibility as low as 50 metres. However, only a handful of airports — notably Delhi — are equipped for such operations. Even then, the aircraft and pilots must be certified for CAT IIIB landings.
Airlines attempt to mitigate disruption by positioning trained pilots and suitably equipped aircraft at fog-prone airports, while keeping reserve crew on standby during the official fog window declared by the Directorate General of Civil Aviation.
Why flight delays still cascade
Technology reduces but does not eliminate fog disruption. Under LVP, aircraft movements are spaced further apart to maintain safety. This sharply cuts airport capacity. At Delhi, hourly aircraft movements can fall from around 100 to nearly 65 during dense fog.
Ground operations slow as well. Taxiing aircraft take longer to reach runways or parking stands, leading to congestion. In some situations, aircraft can land but cannot take off because take-off visibility thresholds are higher than landing thresholds. Parking bays fill up, forcing diversions to alternative airports and triggering network-wide delays.
Because airline operations rely on tight aircraft and crew rotations, disruption at a major hub like Delhi — a key base for IndiGo and Air India — can ripple across the country within hours.
Crew fatigue rules and operational limits
Another constraint is crew safety regulation. Flight Duty Time Limitation norms cap how long pilots and cabin crew can remain on duty, even if they are stuck waiting on the ground. As the has tightened enforcement to address fatigue risks, airlines face crew shortages faster during prolonged fog episodes.
Dynamic crew management and standby rosters help, but beyond a point, cancellations become unavoidable.
Railways: fog as a systemic bottleneck
For the , fog is a seasonal operational stress test. Dense fog in northern India routinely causes train delays stretching beyond 10–12 hours, creating extreme congestion on already saturated routes.
According to Railway Minister Ashwini Vaishnaw, safety remains paramount during fog, even if it means slower movement and cancellations.
Fog safety devices and signalling measures
A key mitigation tool is the Fog Safety Device (FSD), a GPS-based handheld system provided to locomotive pilots in fog-prone sections. It gives audio-visual alerts about signals, stations, level crossings, and speed restrictions even when nothing is visible outside.
By December 2025, nearly 26,000 FSDs had been deployed, with the Northern Railway receiving the largest share. Railways have also introduced modified signalling that limits the number of trains between stations during fog, and luminous strips on signals to improve visibility.
Kavach and the future of fog-proof rail movement
A more structural solution lies in Kavach, India’s Automatic Train Protection system. Kavach displays signal information inside the locomotive cab and automatically applies brakes if the pilot fails to respond, reducing dependence on line-of-sight visibility.
The advanced Kavach 4.0 system is being deployed on high-density corridors such as Delhi–Mumbai and Delhi–Howrah, though coverage remains limited for now. Full-scale implementation could significantly reduce fog-induced slowdowns over time.
What this means for India’s transport strategy
Fog-related disruption highlights the limits of infrastructure-led growth without redundancy. As passenger volumes surge in both air and rail travel, systems are being pushed closer to their operational thresholds. Technology helps, but cannot override physics or safety norms.
Long-term resilience will depend on expanding fog-capable infrastructure beyond a few hubs, accelerating deployment of automatic safety systems, and integrating pollution control with transport planning.
What to note for Prelims?
- Radiation fog and its formation conditions.
- ILS CAT IIIB and Low Visibility Procedures.
- Fog Safety Devices (FSD) and Kavach.
- Role of DGCA in aviation safety.
What to note for Mains?
- Impact of environmental factors on transport efficiency.
- Trade-offs between safety and capacity in public transport.
- Role of technology in mitigating climate-related disruptions.
- Need for integrated pollution and transport policy in urban India.
