For many residents of Delhi, winter means chilly nights, open-air concerts, strolls in the park and maybe a bout of flu.
But for frequent fliers, Delhi's winter also brings a shiver of dread over mangled flight schedules.
When a thick blanket of smog cloaked Delhi on Monday, about a dozen flights were reported to be grounded and scores of others delayed. This time last year, about 30 flights were diverted, 25 cancelled and more than 150 stalled for up to six hours at Delhi's Indira Gandhi International airport. The week leading into 2009 saw the disruption of hundreds of flights.
Could this problem be prevented?Well, yes and no. The Delhi airport is equipped with instrument landing systems that allow for so-called CAT IIIB landings, or mostly automatic landings using a combination of radio signals and high intensity runway lighting, effective even when visibility is as low as 164 feet. These are the most sophisticated systems available in the world, but still, they can't always guarantee a landing.
"If the visibility drops below 50 meters, disruption is unavoidable," said E K Bharat Bhushan, the top official of India's main airline regulatory body, the Directorate General of Civil Aviation. "No one, anywhere in the world, can go completely blind," he said.
Mr Bhushan said the focus is now on making passengers as comfortable as possible by offering refreshments and prompt updates about their flights when they face a shuffled schedule.
But it's not only zero-visibility that is the problem. Analysts say that some aircraft are not equipped with CAT IIIB instruments and many airline pilots aren't trained, largely because of the costs involved, to know how to use these sophisticated instruments and keep their eyes peeled for the landing strip lights.
"Only 40 to 50 percent of pilots are trained," said Rajendra Kumar Jenamani, scientist and head of the meteorological watch office at Delhi airport.
Mr Jenamani also says poor coordination between various parties, including the Delhi International Airport Limited, Air Traffic Control, and the airport's Meteorological Department contribute to disrupted flights. Mr Jenamani cites the example of a flight dispatcher not briefing a pilot about fog gathering over Delhi before take-off from another country, even though the fog was likely to last 10 to 12 hours.
So when did the fog in Delhi get this bad?Analysts say the turning point was 1997. A cocktail of factors contributed: a shift in the land-use patterns, increased pollution, and more moisture due to irrigation led to a drop in the temperature by a few degrees and more fog between December and January, they say.
While fog does plague other places, like London's Heathrow Airport for instance, it lasts only for a few days. In Delhi, it can last for 20 to 25 days. Mr Jenamani said Delhi is hard hit because of the city's geographic location, the open space at the airport, and the prevalent wind patterns.
"Delhi fog is the most notorious," said Mr Jenamani, and it strikes without much warning.
Delhi is hardly unique in India. Cities stretching from Amritsar to Kolkata also battle with fog, thanks to their own geographic conditions and rising pollution. The Delhi airport, technically, is better positioned to handle fog, because it is the only one in India that is capable of hosting CAT IIIB landings. Most other airports in northern India have less powerful systems that can only land aircraft when visibility is better than 300 meters, or 980 feet.
During the past three decades there's been a 5 to 10-fold increase in zero-visibility fog in India, said Mr Jenamani.
So the next time you head to the Delhi airport, don't forget to carry a book and a bottle of water if you're traveling between December and February. Delayed flights look like they may be a long-term problem.