Every winter, Indian travellers packing up for the holiday season face a familiar challenge — fog. Both airlines and the Indian Railways are hit by frequent operational disruptions such as delays and cancellations owing to the low visibility during this time, particularly in northern India.In the case of airlines, technology and specialised operational modalities have blunted the impact of fog to some extent. But the rapid growth in air traveler volumes means that airlines, airports and air traffic controllers are always playing catch-up to minimise the domino effect on operations across the country.When it comes to the Railways, fog-induced disruptions are commonplace, leading to extreme network congestion, long delays and dozens of cancellations.Here’s how both these sectors deal with fog disruptions, especially in Delhi and the other northern parts of the country.First, why is north India’s fog so persistent? The type of fog that affects flight and train operations the most in northern India is radiation fog — which forms under clear winter nights when the ground cools rapidly, moisture condenses and stagnant winds trap it. It can occur frequently and can cause extremely low visibility conditions.Smog adds to the fog’s severity, making it denser and dragging it closer to the ground level. Smog also makes low visibility conditions last much longer than just fog, which dissipates quickly with sunlight. Pilots and officials with Indian carriers say that the high pollution levels have, over the years, compounded fog-related disruptions in northern parts of India, particularly Delhi.How do flights and airports operate in low-visibility?When visibility falls drastically due to fog, airports start operating under Low Visibility Procedures (LVP). These procedures involve specialised operations such as Low Visibility Take-Off (LVTO) and Instrument Landing System (ILS) CAT IIIB landings.Story continues below this adILS is a ground-based radio navigation system at airports that provides precise horizontal and vertical guidance to aircraft for landings.And CAT IIIB, an ILS category, is one of the highest levels for precision approaches for aircraft landings in very low visibility conditions. It allows aircraft to land even when visibility is as low as 50 metres.Not all Indian airports have these capabilities. Among the exceptions are India’s largest hub airport Delhi. At airports that are not equipped for very low visibility operations, flight delays or cancellations are the only viable options if they are hit by dense fog. Besides airports, aircraft and their crew too need to be equipped and certified for CAT IIIB operations.To minimise fog-induced disruptions, airlines try to ensure that an adequate number of pilots trained for low-visibility operations are available to operate flights to and from fog-prone, LVP-equipped airports. The effort is also to have trained crew on standby.Story continues below this adThe carriers also try to station CAT IIIB-certified aircraft at such airports.India’s aviation regulator, the Directorate General of Civil Aviation (DGCA), has announced December 10-February 10 as the official fog window for this season. Airlines and airports monitor real-time meteorological data and use predictive analytics and artificial intelligence tools for better fog prediction.Why some fog disruption is unavoidable for flightsWhile low visibility procedures can help prevent significant disruptions, they still cannot make operations fully immune to fog. Under LVP, the distance and time between aircraft movements — landings and take-offs — increase notably to ensure safe operations.This leads to backlogs in departures and arrivals and adds to airport congestion. Ground movement of aircraft — from taxiways to runways, and the other way round — also slows considerably in very poor visibility. This also leads to delays in the flight schedule and adds to congestion at the airport.Story continues below this adFor instance, while the Delhi airport has the capacity for almost 100 aircraft movements in an hour during regular operations, the capacity drops to 65 during low visibility operations. For every hour of fog-related disruption, the Delhi airport takes about two-three hours to recover.Additionally, there may be a situation where CAT IIIB landings are taking place, but the visibility is not conducive for low visibility take-offs. This is because the minimum visibility required for a take-off by a CAT IIIB-equipped aircraft at an airport like Delhi is 75 metres, against 50 metres for a CAT IIIB landing.In such situations, the affected airport’s parking stands can quickly get congested as aircraft would continue to land while those waiting to depart remain stuck. This sort of congestion can also lead to flight diversions.And if visibility drops to under 50 metres, even CAT IIIB might not be enough for safe operations, and landings might have to be halted till visibility improves.Story continues below this adThe airlines plan fair-weather alternative airports for fog-prone airports, in case flights have to be diverted. For instance, a fair-weather alternative for a flight being diverted from the Delhi airport would be Ahmedabad airport as the chances of fog at the latter would be minimal. However, a decision on where to divert is taken based on the real-time weather and visibility situation at nearby airports. The carriers also try to take into account only those alternative airports where their engineering staff is stationed.The challenge for airline networks: cascading impact Severe fog-induced disruptions don’t just affect operations at a fog-hit airport, but can have a cascading impact across the carrier’s network. This is most likely to happen when operations at a major airport like Delhi — a major hub for India’s biggest airlines IndiGo and Air India — are disrupted.Airline networks and operations are complex. An aircraft and a set of crew works on multiple flights on different sectors through the day. And delays and cancellations could continue to build up throughout the day for that aircraft.In instances of such disruptions, it is airlines with large and dense networks and high aircraft and crew utilisation levels that tend to suffer more. While such parameters reflect efficiency in operations, finding and deploying spare aircraft and crew at a short notice can be a huge challenge.Story continues below this adAnother factor that contributes to the cascading impact are crew rest and duty duration rules. The Flight Duty Time Limitation (FDTL) guidelines include limitations on duty time as well as flight time. This means that if pilots and crew exceed the maximum permissible duty time for the day — even if they have not been able to operate any flight in that period — they have to be taken off duty.The Directorate General of Civil Aviation (DGCA) has increasingly turned stricter on FDTL compliance as fatigue is a major risk factor in aircraft operations.With time wasted awaiting take-off or finding a gate after arrival during acute congestion at airports, in addition to previous delays due to low visibility, crew shortage can hit airlines fairly quickly due to the FDTL norms in case of large-scale disruptions. In such cases, crew rosters go haywire and airlines are left scrambling for crew to operate flights.This is why airlines try to have more crew on standby during the fog window, but even that has its limits. Airlines also initiate dynamic crew management during fog disruptions to ensure FDTL compliance.Story continues below this adNow, how does the Railways manage train operations during fog?The winter season always remains a challenge for the Indian Railways as it operates trains during the foggy weather, especially the northern part of the country.Due to this, the trains are delayed even for more than 12 hours in many cases. To overcome this problem, Railways has been taking up multiple measures, including the provisioning of fog safety devices (FSD) in trains and deployment of Automatic Train Protection System.Fog safety device or Fog pass device is a GPS-based, handheld system provided to locomotive pilots in the -affected sections. It alerts the train driver through an audio-visual alarm if it detects anything within the geo-fenced range. It provides a variety of other information such as on stations, warning boards, signals and level crossing gates.In December 2025, the Minister for Railways told Rajya Sabha that a total of 25,939 fog safety devices have been provisioned for the season. Almost 23% of the total FSDs go to the Northern Railway alone, as it operates the most trains in the most fog-prone regions.Story continues below this adApart from this, a modified automatic signalling system has been introduced in Northern Railway and North Central Railway which restricts the number of trains to two between two stations during fog. The Railways has introduced luminous strips on signalling equipment to improve visibility.The Railways is also implementing its ambitious Automatic Train Protection (ATP) system — Kavach — in the select sections of the network.This system will ensure that locomotive pilots can maintain their train’s speed even during dense fog. They would not even have to look outside the cabin for a signal as the relevant information is flashed on the dashboard installed inside the locomotive.Kavach also applies brakes automatically in case a pilot fails to do so.The advanced version of this system — Kavach 4.0 — is planned for large-scale deployment over the Railways network. As of now, Kavach 4.0 has been commissioned on a total of 738 route km, comprising the Palwal-Mathura-Nagda section (633 route km) of Delhi-Mumbai corridor and Howrah-Bardhaman section (105 route km) of Delhi-Howrah corridor. The work is still on in the remainder of the sections of these two high-density corridors.