Air quality in Delhi and its surrounding areas improved marginally due to rain and favourable wind speed under the influence of a western disturbance affecting northwest India.

The Safdarjung Observatory, the national capital's primary weather station, recorded 7.2 mm of rainfall till 8:30 pm on Monday.

The wind speed improved to 20 kilometres per hour, aiding the dispersion of pollutants, an official at the India Meteorological Department said.

Delhi's air quality index (AQI) stood at 365 at 8 am on Tuesday, improving from 395 at 4 pm on Monday. The AQI at any given time is the average of readings taken in the last 24 hours.

The 24-hour average AQI, recorded at 4 pm every day, was 395 on Sunday, 389 on Saturday, 415 on Friday, 390 on Thursday, 394 on Wednesday, 365 on Tuesday, 348 on Monday and 301 on November 19.

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'.

Earlier on Monday, a thick layer of smog blanketed Delhi, reducing visibility to just 600 metres at the Safdarjung Observatory. The visibility was 800 metres at the Indira Gandhi International Airport.

The city recorded just three severe air quality days in November last year, while it experienced 12 such days in 2021, the maximum in the month since the Central Pollution Control Board (CPCB) began monitoring.

There were nine such days in November 2020, seven in 2019; five in 2018; seven in 2017; 10 in 2016, and six in 2015, according to CPCB.