Low wind speeds coupled with an increase in cases of stubble burning in the neighbouring states of Punjab and Haryana pushed the air quality to “severe” for a major part of Thursday.

The 24-hour rolling average of the Central Pollution Control Board (CPCB) recordings at 4pm showed that while the overall air quality index (AQI) in Delhi settled at 395 — in the “very poor” category — the hourly average AQI was above the “severe” mark for most part of the day.

Till 1pm, the average AQI oscillated between 402 and 408. At monitoring stations in Mundka, Bawana, Narela, Wazirpur and Anand Vihar, pollution levels neared the 500 mark, the worst AQI recordings since January. On October 23, AQI reached perilously close to the “severe” zone with a reading of 366.

An AQI of 201 to 300 is considered “poor”. A reading of 301 to 400 is in the “very poor” category and is associated with respiratory illnesses, especially in children and those exposed to the bad air. An AQI of 400-500 is considered “severe”.

Scientists attributed the bad air to calm surface wind conditions that have led to the accumulation of pollutants in the atmosphere and stubble burning in Punjab and Haryana that is affecting the National Capital Region (NCR).

VK Soni, head of the environment monitoring research centre of the India Meteorological Department (IMD), said the wind speed was below 4kmph on Thursday. He added that the situation was expected to improve from Friday.

“The wind speed through the day was below 4kmph. But the situation will improve from tomorrow (Friday). By Saturday, there will be a significant improvement in the air quality,” Soni said.

He said wind speed will pick up to 8-12kmph on Friday. And by Saturday, the wind speed will be around 15-18kmph.

Satellite images from the National Aeronautics and Space Administration (Nasa) showed a thick cover of smoke over the Delhi-NCR region on Thursday.

The System of Air Quality and Weather Forecasting and Research (Safar) of the Union ministry of earth sciences said stubble fires spotted over Punjab and Haryana were the highest (2,912) this season on Wednesday and their share (36%) in the city’s PM 2.5 (ultrafine particulate matter with diameter less than 2.5 micrometres) levels too hit a new high on that day.

Pawan Gupta, a senior scientist of earth sciences at the Science and Technology Institute (STI), Universities Space Research Association (USRA), said the number of farm fires spotted over Punjab and Haryana are likely to have crossed the 2,000 mark for the third consecutive day on Thursday.

“It appears we are entering the heavy burning period. The satellite recording shows that the stubble fires crossed the 2000 mark for the third consecutive day. The impact of this can be seen in the thick haze over the northern India,” Gupta said.

Low temperatures on Thursday also led to the accumulation of pollution particles closer to the surface. The national capital recorded a minimum temperature of 12.5 degrees Celsius on Thursday, the lowest in October in 26 years. IMD recordings showed that the maximum temperature in the Safdarjung observatory, which is considered the official reading of the city, was 32.2 degrees Celsius.

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