Delhi's air quality slipped into the 'very poor' category on Thursday, a day after it was recorded in the 'poor' category.

The national capital recorded a maximum temperature of 25.1 degrees Celsius, a notch below the season's average, while the minimum temperature settled two notches above normal at 12.6 degrees Celsius.

Delhi's 24-hour average Air Quality Index (AQI) was recorded in the 'very poor' category with a reading of 398.

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'.

Delhi's 24-hour average AQI was recorded in the 'poor' category with a reading of 290 on Wednesday.

The air quality is likely to remain 'very poor' over the next six days.

The 24-hour average AQI -- recorded at 4 pm every day -- was at 395 on Monday and Sunday, 389 on Saturday, 415 on Friday and 390 on Thursday (November 23).

The humidity levels oscillated between 66 per cent and 100 per cent.

The India Meteorological Department (IMD) has forecast partly cloudy skies for Friday with the possibility of shallow fog in the morning.

The maximum and minimum temperatures are likely to settle around 25 and 13 degrees Celsius, respectively.

The IMD said shallow fog may be expected over the next four days.

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