New Delhi, Dec 27 : Delhi woke up to a blanket of dense fog on Wednesday, significantly reducing the visibility across the city to less than 50m affecting flights, rail and road traffic. The India Meteorological Department (IMD) classifies fog intensity into four types -- shallow, moderate, dense and very dense fog. The visibility ranges from 999m to 500m, 499m to 200m, 199m to 50m and < 50m, respectively. As the day unfolds, the fog is forecasted to gradually dissipate, revealing a clearer atmosphere by the afternoon. The dense fog has led to increased caution in railway operations, affecting the departure and arrival times of trains. Flight schedules are also being affected, with many flights experiencing delays and some possibly facing cancellations. As of 8 a.m., more than 40 flights have been delayed due to the ongoing dense fog. This includes disruptions in eight international departures, four international arrivals, 22 domestic departures and five domestic arrivals. Delhi airport has also issued an advisory, which reads: "While landing and takeoffs continue at Delhi Airport, flights that are not CAT Ill compliant may get affected. Passengers are requested to contact the airline concerned for updated flight information. Any inconvenience caused is deeply regretted." In a fog-induced challenge for commuters, several trains arriving in the Delhi region are also dealing with delays, causing frustration among passengers. 25 trains are trailing behind their scheduled arrival time, and the extent of delay varies across multiple routes. Puri-New Delhi Purushottam Express and Howrah-New Delhi Poorva Express are delayed by five hours, which is also the maximum number of hours delayed throughout the day. Passengers on or about to board Chennai-New Delhi Grand Trunk Express were also urged to check updated schedules due to delays of 4:40 hours. Among 25 trains delayed, the Golden Temple Mail is also running behind schedule by 2: 38 hours. Delhi's air quality remained in the 'severe' category. The AQI scale categorises levels from 0 to 500, with Anand Vihar breaching the 'severe' threshold. At the Anand Vihar area, PM2.5 levels remained into the 'severe' category standing at 500 and PM10 reached 500, and CO plummeted to 118, classified as 'moderate'," according to the Central Pollution Control Board (CPCB). The Air Quality Index (AQI) between zero and 50 is considered 'good'; 51 and 100 'satisfactory'; 101 and 200 'moderate'; 201 and 300 'poor'; 301 and 400 'very poor'; and 401 and 500 'severe'. The Indira Gandhi International (IGI) Airport T3 witnessed PM2.5 levels at 437 and PM10 at 431, both falling under the 'severe' category. The CO level was recorded at 132, maintaining a 'moderate' rating. Commuters are urged to exercise caution during the morning hours due to reduced visibility caused by the lingering fog, and are urged to stay updated on real-time schedules and plan their journeys accordingly. /IANS