Why Winter Fog Brings India’s Trains and Flights to a Crawl — and How Technology Tries to Cope

Every winter, dense fog becomes an unavoidable disruptor of India’s transport networks. From grounded flights to trains running hours behind schedule, low visibility — especially across northern India — triggers cascading delays that affect millions of travellers. While airlines and airports have partially tamed fog through advanced navigation and procedures, the scale and complexity of India’s rail network make fog-related disruptions far harder to manage.

Why north India’s fog is so persistent

The fog that paralyses transport in northern India is largely radiation fog. It forms on clear winter nights when the ground cools rapidly, moisture condenses near the surface, and calm winds trap the fog close to the ground. What makes this fog especially disruptive is its frequency and intensity — visibility can drop to a few tens of metres for hours at a stretch.

Air pollution compounds the problem. Smog mixes with fog, making it denser, longer-lasting, and slower to disperse even after sunrise. Aviation professionals say that rising pollution levels around cities like Delhi have worsened winter visibility over the years, extending the duration and severity of disruptions.

How airports and airlines operate in dense fog

When visibility drops sharply, airports activate Low Visibility Procedures (LVPs). These include Low Visibility Take-Offs and precision landings using the Instrument Landing System (ILS). Among the most advanced is ILS Category IIIB, which allows aircraft to land with visibility as low as 50 metres.

Not all airports are equipped for such operations. Major hubs like “Indira Gandhi International Airport” have CAT IIIB capability, but many smaller airports do not — forcing delays or cancellations when fog strikes.

Crucially, airports alone are not enough. Aircraft must be certified for CAT IIIB operations, and pilots need specialised training. Airlines therefore roster fog-trained crews and position CAT IIIB-capable aircraft at fog-prone airports during the winter “fog window”, which India’s aviation regulator — the “Directorate General of Civil Aviation” — officially defines as December 10 to February 10.

Why fog still disrupts flights despite advanced technology

Even with LVPs, fog cannot be fully neutralised. Safety protocols require greater spacing between aircraft during take-offs and landings, sharply reducing airport capacity. At Delhi, hourly aircraft movements can fall from nearly 100 in normal conditions to about 65 during low visibility.

Ground operations slow as well — taxiing to and from runways becomes painstakingly slow in near-zero visibility. For every hour of severe fog disruption, Delhi airport can take two to three hours to recover.

There are also asymmetries in operations. CAT IIIB landings are permitted at 50 metres visibility, but take-offs usually require at least 75 metres. This can result in aircraft continuing to land while departures remain stuck, quickly clogging parking bays and forcing diversions.

The cascading impact on airline networks

Fog at a major hub like Delhi does not stay local. Airlines such as “IndiGo” and “Air India” operate dense networks where the same aircraft and crew fly multiple sectors a day. Delays at one hub ripple across the country.

Crew duty limits add another layer of complexity. Under Flight Duty Time Limitation (FDTL) rules enforced by the DGCA, pilots and cabin crew must be taken off duty once they exceed prescribed limits — even if they have spent the day waiting on the tarmac. During prolonged fog disruptions, airlines can quickly run out of legally deployable crew, triggering cancellations far beyond the fog-hit airport.

Why railways face a tougher fog challenge

For the “Indian Railways”, fog is an even bigger operational headache. Northern routes frequently see trains delayed by six to twelve hours, creating severe network congestion.

Unlike aviation, where traffic can be throttled or diverted, railways operate on fixed tracks with tightly sequenced movements. Reduced visibility directly affects signal sighting and braking distances, forcing trains to slow down dramatically.

Fog safety devices and signalling restrictions

To mitigate risks, Railways deploy Fog Safety Devices (FSDs) — GPS-based handheld units given to loco pilots in fog-prone sections. These devices issue audio-visual alerts about stations, signals, level crossings, and warning boards within geo-fenced zones.

In December 2025, Railway Minister “Ashwini Vaishnaw” informed Parliament that nearly 26,000 FSDs had been provisioned for the season, with the largest share allocated to Northern Railway.

Railways have also modified signalling systems in some zones, limiting the number of trains between stations during fog, and added luminous strips to improve signal visibility.

Kavach: the long-term technological fix

The most ambitious solution lies in Automatic Train Protection. Railways are gradually rolling out Kavach, an indigenous ATP system that displays signal information inside the locomotive cab and automatically applies brakes if the driver fails to respond.

An advanced version, Kavach 4.0, is being deployed on high-density corridors. It has already been commissioned on parts of the Delhi–Mumbai and Delhi–Howrah routes, covering over 700 route kilometres. Once fully implemented, Kavach could allow trains to maintain higher speeds safely even in dense fog, reducing delays without compromising safety.

Technology helps — but fog still sets the limits

Despite advances in aviation and rail technology, fog remains a powerful constraint. Safety margins inevitably reduce capacity, and recovery takes time. As passenger volumes continue to grow rapidly, both airlines and railways find themselves constantly playing catch-up.

The lesson each winter is the same: technology can blunt fog’s impact, but it cannot eliminate it. Until air quality improves and networks gain greater redundancy, fog will remain an annual stress test for India’s transport systems.