A thick cover of fog engulfed the national Capital on Saturday morning, resulting in delay and cancellation of train and flight services and disruption in traffic movement, reported PTI. The India Meteorological Department said that visibility reduced to zero in Delhi.

The IMD classifies “very dense” fog when visibility is between 0 and 50 metres, and “dense” fog when visibility is between 51 and 200 metres. Saturday marked the third time this season, after December 8 and January 1, that visibility has dropped to zero in the city, according to PTI.

Around 80 flights departing from and over 50 flights bound to Delhi airport were delayed because of the fog, while at least one them was cancelled, ANI reported.

“Due to dense fog, only CAT IIIA and CATIII B compliant aircraft and pilots are able to operate,” the airport tweeted in the wee hours of Saturday. “Passengers are requested to contact the airline concerned for updated flight information.”

The cold weather also persisted in the Capital, as Delhi recorded a minimum temperature of 6.6 degrees Celsius, a notch below normal, on Saturday.

Meanwhile, the city’s Air Quality Index entered the “severe” category, with a reading of 436 at 9 am, according to PTI. The 24-hour average AQI was 460 on Friday and 429 on Thursday. An 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’’.

Kuldeep Srivastava, the head of the IMD’s regional forecasting centre, said that the wind speed has slowed down, which along with moisture in the air has made the pollutants heavier. The central government’s Air Quality Early Warning System warned that further deterioration of the air quality is expected in the next few days, PTI reported.