A blanket of fog covered parts of Delhi for the second consecutive day on Thursday with the air quality index remaining in the 'severe' category, an official said.

The city recorded its lowest minimum temperature of the season so far at 16.1 degrees Celsius, three notches above the normal, according to the weather department.

According to the Central Pollution Control Board, Delhi's Air Quality Index at 9 am was 428, which falls in the 'severe' category.

"Of 39 monitoring stations in Delhi, 32 recorded air quality in the 'severe' category, with readings above 400. These stations include Anand Vihar, Ashok Vihar, IGI Airport, ITO, Mandir Marg, North Campus, Patparganj, Punjabi Bagh, and Pusa among others," an official said.

The air quality in Delhi on Wednesday was reported as the worst in the country, plunging into the 'severe' category for the first time this season.

The city's 24-hour AQI, recorded at 4 pm every day, stood at 418 on Wednesday, up from 334 the previous day.

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", 401 and 450 "severe" and above 450 "severe plus".

A thick veil of fog blanketed the city on Wednesday morning, reducing visibility to zero and affecting flight operations. The situation improved slightly on Thursday as the visibility at the Delhi Airport at 8.30 am was 400 metres.

The weather department said the maximum temperature is expected to settle at around 29 degrees Celsius.

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