Delhiites woke up to a gloomy morning as a blanket of dense fog enveloped the national capital and the air quality dropped to “severe” category, resulting in a delay in flights and train services on Saturday.

The Air Quality Index worsened as it rose to 492, as opposed to Friday’s 460, leading to low visibility.

Meanwhile, flight operations have been restricted to CAT IIIA and CAT IIIB aircraft due to the low visibility. The Delhi Airport, in a tweet on Saturday morning, urged passengers to keep an eye out on updates. “Only CAT IIA and CAT IIB compliant aircraft and pilots are able to operate. Passengers are requested to contact the Airline concerned for updated flight information,” it said.

On Friday, Delhi's minimum temperature rose to 6.4 degrees Celsius, a notch above normal, due to partly cloudy weather, the India Meteorological Department (IMD) said. The maximum temperature stood at 20.8 degrees Celsius, a notch above normal.

Moderate fog lowered visibility to 201 metres at Safdarjung and 300 metres at Palam in the morning, an IMD official said. Dense fog is predicted in parts of the city on Saturday. According to the IMD, very dense fog is when visibility is between 0 and 50 metres. In case of dense fog, the visibility is between 51 and 200 metres, moderate at 201 and 500 metres, and shallow at 501 and 1,000 metres.

On Thursday, the Safdarjung Observatory, which provides representative data for the city, recorded a minimum of 2 degrees Celsius, five notches below normal. It was 3.2 degrees Celsius on Wednesday. Cold and dry northerly/northwesterly winds from the western Himalayas brought the minimum temperature down in Delhi on Thursday. The wind direction then changed to northeasterly. This, along with partly cloudy weather, resulted in an increase in the minimum temperature, said Kuldeep Srivastava, the head of the IMD's regional forecasting centre.

The city's 24-hour average air quality index (AQI) was 460. It was 429 on Thursday, 354 on Wednesday, 293 on Tuesday and 243 on Monday. An AQI between zero and 50 is considered good, satisfactory at 51-100, moderate at 101-200, poor at 201-300, very poor at 301-400, severe at 401-500.