Without CAT III capability, a fog-prone hub’s operating floor sits far higher than the 50 metres of Runway Visual Range a CAT IIIB approach can handle. When visibility drops below the airport’s limit, landings stop, diversions stack and cancellations begin — and clearing the backlog can take days, even after the fog lifts.
Delhi’s CAT IIIB-equipped airport still lost airlines about $3.9 million across 653 dense-fog hours between 2011 and 2016, according to a study published by MDPI in the journal Atmosphere. That is the number for a hub that keeps working; the gap is far bigger where the certification does not exist.
Fog is treated like one weather event, but for an airport it is a stack of thresholds. At 550 metres of runway visual range, a standard CAT I approach still works. At 200 metres, a lot of secondary hubs stop accepting landings. At 50 metres, a properly trained crew in a redundant aircraft can touch down on autopilot. The space between those numbers is where cancellations, diversions and network recovery are decided.
Delhi’s main airport has the instruments to operate near the bottom of that range — and still its fog seasons produce hundreds of diversions and cancellations. Now consider the fog-prone airports in the same region that lack that capability: a higher limit, a sooner shutdown, a longer tail. Some of them stay that way for reasons a technical explainer will never reach — shared runways with air forces, retrofit economics for budget carriers, and a certification stack that has to line up across crew, aircraft and runway at the same time.
What a fog morning costs at the gate
The closest thing to a complete fog-economy record comes from Delhi — a CAT IIIB hub, not a worst-case airport. The worst single winter, 2013–2014, produced 363 delays, 143 diversions and 141 cancellations across about 160 fog hours, with airline losses near $1.78 million.
Then regulators tightened the rules. After strict CAT III guidelines, delays dropped roughly 88%, diversions 55% and cancellations 36% at Delhi, and the following winter’s losses fell by about 65% to $0.675 million. Same airport, same fog season, sharply different outcome — that delta is what the certification insures.
The trouble is the data ends where the weakest airports begin. No 2025–2026 dataset isolates fog-driven losses at non-CAT III secondary hubs in India. The published figures describe a hub that keeps operating; they cannot tell you what a shutdown at a place without low-visibility capability costs.
Fog losses travel. A December 2006 freezing-fog episode in the UK stranded about 175,000 passengers across seven airports, with British Airways’ bill initially estimated at no less than £25 million. European research puts a one-day weather-forced airport closure at around 435 million euros in airline losses. Once the airfield reopens, the backlog compresses recovery into days, not hours.
Three approvals stand between a flight and the runway
A CAT III approach never clears solely because the airport can handle it. Three things have to line up on the day: the flight crew must be trained for low-visibility operations, the aircraft must carry redundant systems, and the runway itself must meet the lighting standard. Inside the autoland system, triple-redundant channels compare data constantly so one failed component cannot push the aircraft off course.
The night of 5–6 January 2014 at Delhi shows how hard that stack is to assemble. The CAT IIIB ILS stayed fully operational while visibility sat below 150 metres from 20:30 to 02:30 IST — yet 52 flights diverted because many inbound aircraft were not CAT III-compliant. The runway was ready; the crews and airframes were not.
That is the silent vulnerability: even at a fully equipped airport, one uncertified crew or aircraft is enough to ground a flight in fog. Regulators knew it — joint DGCA, IMD and AAI guidelines mandated CAT IIIB compliance for all arrivals into Delhi during the vulnerable winter window, because the tower cannot assume the rest of the stack exists.
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Why some fog-prone airports never get the upgrade
Not every fog-prone runway can simply buy the fix. At Chandigarh, a joint-use civil-military airfield, the Department of Military Affairs told the Punjab and Haryana High Court that fitting CAT III-B ILS would “entail demolition and relocation of most of functional infrastructure of a military airfield as well as national air defence assets” and be detrimental to operational needs. When the runway belongs to both civilian operators and the air force, the certification decision becomes a shared-control problem.
The budget-airline side is just as unsettled. Public fog-season coverage for IndiGo and other low-cost carriers lists cancellations and network strain, but no priced example exists of a fleet-wide autoland retrofit — no per-aircraft budget, no published business case. Whether a budget carrier would recover the investment, or whether it would harden a two-tier operational reality, remains an open industry question.
Cost is the pressure point, and it is steep even where the airspace is uncomplicated.
The ground is the harder problem
Landing is only half the job. Even when an aircraft touches down on autopilot, getting it to the gate in dense fog still depends on human eyes and manual help. Taxiing in low visibility forces wide gaps between aircraft and slow, controlled movement — often with follow-me vehicles leading the way.
European guidance splits this into graded conditions. Condition 3 and Condition 4 cover runway visual range below 400 metres and down to 75 metres, using A-SMGCS surveillance, automated centreline lighting, stop bars and routing. In contrast, Indian documentation for fog-prone Hyderabad triggers low-visibility procedures below 750 metres, allows CAT I down to 550 metres, and stops everything below 200 metres.
That gap is the whole story. The most advanced CAT IIIC ground systems could theoretically move aircraft in zero visibility, but commercial operations have never adopted them — so at many airports the follow-me vehicle remains the manual reality, and the taxiway becomes the real bottleneck.
Booking around the fog season
For a traveler connecting through Asia-Pacific in winter, the question is not what the weather is doing at origin or destination. It is what the hub can handle. Delhi’s CAT IIIB minima sit at 50 metres; a secondary airport can stop accepting flights at 200 metres or more. The difference is the gap between a delayed morning and a day of missed connections.
Chandigarh showed the downside in one January 2026 fog day: 26 flights cancelled and 28 delayed at a single airport. If your itinerary crosses North India between December and February, build longer layover buffers, and on tight business schedules prefer connections through hubs with lower operating minima. A cheaper fare through a secondary airport carries a hidden disruption risk that the fare rules will not refund.
Questions? Answers.
Do flights get cancelled due to fog?
Yes. Fog regularly forces cancellations at fog-prone airports. Delhi’s 2013–2014 winter produced 141 cancellations across about 160 dense-fog hours, and a single fog day at Chandigarh in January 2026 saw 26 flights cancelled.
Does fog interfere with flights?
Yes. Fog reduces visibility, forces wider aircraft spacing, and triggers delays, diversions and cancellations. Delhi logged 363 delays, 143 diversions and 141 cancellations in one winter, and a 2006 UK fog episode stranded about 175,000 passengers across seven airports.
What are CAT III conditions?
CAT III conditions are the most restrictive low-visibility operating bands used for approaches and ground movement. European guidance splits surface movement into Condition 3, for runway visual range below 400 metres, and Condition 4, down to 75 metres; Indian guidance for Hyderabad triggers low-visibility procedures below 750 metres and halts operations below 200 metres.
How foggy is too foggy to fly?
The cutoff depends on what the airport, crew and aircraft are certified for. At Hyderabad, all operations stop once runway visual range drops below 200 metres; at airports without low-visibility capability, the effective floor is higher, as a January 2026 Chandigarh fog day showed with 26 cancellations and 28 delays.
