It took 14 hours and a drastic change in wind direction for Delhi’s air to turn from moderate — a rarity in the month of November — to severe.
At 4 pm on Sunday, Delhi’s air was moderately polluted, with the air quality index value reading at 171. On Monday, this reading was 426. The reason, according to scientists at System of Air Quality and Weather Forecasting and Research (SAFAR), is the change in wind direction and stubble burning to the northwest of Delhi. According to an analysis by SAFAR, stubble burning contributed to 33 per cent of pollution on Monday.
Between Friday and Sunday, the direction of the wind was from the northeasterly direction. On early Monday morning, the direction changed to northwesterly. “There has been a significant intrusion of biomass-(stubble) generated pollution (33%)… which may decline rapidly now as upper winds are slow… Wind is still coming from North West region which is unfavourable,” a statement issued by SAFAR scientists said.
Scientists have forecast a relatively cleaner Tuesday, when the air quality is expected to be very poor. “Although there is a lot of moisture introduced in the air (which aids in accumulation of pollutants) after the withdrawal of westerly disturbances, it may dry faster as no significant drop in temperature has been observed. The wind speed which was 29kmph at transport height (the height at which pollutants can be transported over a considerable distance) is declining now. Pollution likely to get stabilised,” it added.
The share of stubble burning in pollution is also expected to reduce to 14 per cent by Tuesday.
According to several studies, crop-residue burning in Punjab and Haryana, coupled with the flow of wind in the northwesterly direction, makes the last two weeks of October and first week of November very polluted. In addition to these factors, the increase in humidity and dip in temperature at this time also aid the accumulation of pollutants.