New Delhi, Jan 16 : A thick layer of fog blanketed Delhi as the city continued to grapple with cold conditions on Tuesday morning with minimum temperature of 3.5 degrees Celsius, four notches below season's average. For the past four days, the minimum temperature has been hovering around 3 to 4 degrees Celsius, and the cold wave conditions are likely to continue till Wednesday. According to India Meteorological Department (IMD)'s forecast, the minimum temperature is going to be 3 degrees Celsius and the maximum might hover around 21. The IMD has also issued an orange alert for very dense fog, which has also affected the air and land traffic leading to several delays in flights and trains. Over 70 flights have been delayed and many were reported to have been cancelled. An advisory has also been issued for the travellers, which said, "While landing and takeoffs continue at the Delhi airport, flights that are not CAT III compliant may get affected. Passengers are requested to contact the airline concerned for updated flight information. Any inconvenience caused is deeply regretted." Moreover, 30 trains have been delayed with the maximum number of hours being more than six. At 7 a.m., 100 m visibility was reported at the Palam Airport and 00 m half an hour later, whereas Safdarjung Airport recorded 50 m visibility. The IMD classifies fog intensity into four types: shallow, moderate, dense and very dense fog. The visibility ranges from 999 m to 500 m, 499 m to 200 m, 199 m to 50 m and less than 50 m, respectively. The Air quality continued to remain in the 'very poor' category at several stations across the city. In the Anand Vihar area, PM2.5 levels were recorded to be in the 'severe' category standing at 424 and PM10 reached 245 or 'poor', 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 also witnessed PM2.5 levels at 420, the 'severe' category while the PM10 was at 375, falling under the 'very poor' category. /IANS