<p> Delhi's air quality was recorded in the “poor” category on Thursday morning and is likely to deteriorate further amid a dip in the minimum temperature.</p>.<p>A change in the wind direction has also increased the share of stubble burning in the city's pollution slightly.</p>.<p>Delhi's air quality index was recorded at 272 at 9 am. The 24-hour average AQI was 211 on Wednesday and 171 on Tuesday.</p>.<p>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".</p>.<p>V K Soni, the head of the IMD's environmental research centre, said the northwesterly winds increased the stubble contribution slightly on Wednesday.</p>.<p>According to the Ministry of Earth Sciences' air quality monitor, SAFAR, stubble burning accounted for 8 per cent of Delhi's PM2.5 pollution on Wednesday. It was 3 per cent on Tuesday.</p>.<p>Around 800 fire points were observed in Punjab, Haryana and western Pakistan on Wednesday. However, their impact on Delhi-NCR's air quality will not be significant, Soni said. </p>.<p>The central government's Air Quality Early Warning System for Delhi also said Delhi-NCR's air quality is likely to remain in the “moderate” and the lower end of the “poor” category on Thursday and Friday.</p>.<p>However, SAFAR said it may deteriorate to the lower end of the “very poor” category on Friday and Saturday, “as the conductive meteorological environment created after rainfall is slowly retreating”.</p>.<p>The minimum temperature settled at 9.4 degrees Celsius on Thursday – the lowest this season so far -- and the maximum wind speed was predicted to be 10 kmph, according to IMD.</p>.<p>Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion. </p>.<p>Delhi's ventilation index -- a product of mixing depth and average wind speed – was around 5,000 m2/s on Wednesday and is likely to be 7,500 m2/s on Thursday.</p>.<p>Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.</p>.<p>A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.</p>.<p>The IMD had earlier said the minimum temperature in Delhi will drop to 9 degrees Celsius by Saturday, as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.</p>.<p>The minimum temperature this month, barring on November 16, has remained 2-3 degree Celsius below normal in the absence of a cloud cover, according to IMD officials.</p>
<p> Delhi's air quality was recorded in the “poor” category on Thursday morning and is likely to deteriorate further amid a dip in the minimum temperature.</p>.<p>A change in the wind direction has also increased the share of stubble burning in the city's pollution slightly.</p>.<p>Delhi's air quality index was recorded at 272 at 9 am. The 24-hour average AQI was 211 on Wednesday and 171 on Tuesday.</p>.<p>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".</p>.<p>V K Soni, the head of the IMD's environmental research centre, said the northwesterly winds increased the stubble contribution slightly on Wednesday.</p>.<p>According to the Ministry of Earth Sciences' air quality monitor, SAFAR, stubble burning accounted for 8 per cent of Delhi's PM2.5 pollution on Wednesday. It was 3 per cent on Tuesday.</p>.<p>Around 800 fire points were observed in Punjab, Haryana and western Pakistan on Wednesday. However, their impact on Delhi-NCR's air quality will not be significant, Soni said. </p>.<p>The central government's Air Quality Early Warning System for Delhi also said Delhi-NCR's air quality is likely to remain in the “moderate” and the lower end of the “poor” category on Thursday and Friday.</p>.<p>However, SAFAR said it may deteriorate to the lower end of the “very poor” category on Friday and Saturday, “as the conductive meteorological environment created after rainfall is slowly retreating”.</p>.<p>The minimum temperature settled at 9.4 degrees Celsius on Thursday – the lowest this season so far -- and the maximum wind speed was predicted to be 10 kmph, according to IMD.</p>.<p>Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion. </p>.<p>Delhi's ventilation index -- a product of mixing depth and average wind speed – was around 5,000 m2/s on Wednesday and is likely to be 7,500 m2/s on Thursday.</p>.<p>Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.</p>.<p>A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.</p>.<p>The IMD had earlier said the minimum temperature in Delhi will drop to 9 degrees Celsius by Saturday, as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.</p>.<p>The minimum temperature this month, barring on November 16, has remained 2-3 degree Celsius below normal in the absence of a cloud cover, according to IMD officials.</p>