By 7am on Friday, the Central Pollution Control Board’s (CPCB) hourly AQI had climbed to 389, 20 points above the 369 in Thursday’s 4pm bulletin. Delhi’s air got worse through Thursday night. The wind near the ground fell almost still, and a layer of warm air held the night’s pollution close to the city.
Winds at that height are known as transport-level winds, and they carry smoke over long distances. Those numbers understate the burning. Nasa’s satellite passes over in the early afternoon, and farmers have been burning later in the day each year, after it has gone. Near the ground, the air barely moved. Air usually cools with height, which lets pollution rise and spread out. A warm layer above cooler air, called a temperature inversion, works like a lid and holds pollution near the ground. Afternoon sunshine usually helps clear the air. It heats the ground, and the rising warm air carries pollution into a deeper layer, where it thins out.
Higher up, the wind blew in from the north-west, from Punjab and Haryana, where Nasa’s satellite counted 226 farm fires on Thursday. The India Meteorological Department (IMD) sent up a weather balloon from Safdarjung at 4.30am, and it measured winds of 23 km/h between 500 and 1,500 metres above the ground. The satellite’s daily count of fires in the two states has nearly doubled in a week, from 120 on October 30 to 246 on both November 3 and 4, according to the HT Pollution Dashboard’s tally of Nasa data. Since October 1, it has detected 2,477 fires, 2,232 of them in Punjab. CPCB’s hourly figure for each station is a running average of the past 24 hours, updated every hour. An HT Pollution Dashboard analysis of those readings shows 18 severe stations at 4pm on Thursday, rising to 28 by 5am, the most at any hour in the past week. ITO, Mandir Marg, JNU and Okhla Phase-2 were among the 10 that crossed 400 overnight. Safdarjung airfield logged calm air in seven of its 10 weather reports between 8pm and 6.30am, and the wind averaged 1 km/h, well short of the 15 km/h or more that helps clear Delhi’s air. By 7.30am, it had picked up only to 4 km/h. The 4.30am balloon also found a layer of air about 250 metres above the ground that was 6°C warmer than the surface, slightly warmer than on Thursday morning. PM2.5, or fine particles, was the main pollutant at 53 of the 59 stations. Thursday’s bulletin averages readings over the 24 hours to 4pm. It put Delhi’s AQI at 369, the fourth worst among the 283 cities CPCB reported. On Thursday afternoon that layer reached only about 850 metres, against about 1,300 metres usual for early November, though the wind through it was close to normal.
At 7am, 28 of Delhi’s 59 reporting stations were in the “severe” category, which means above 400 on CPCB’s 500-point scale. IIT Delhi and Rohini were the worst, at 474 each, followed by Punjabi Bagh (465) and Jahangirpuri (460), and every station in the city was “very poor” or worse.
Friday’s 4pm bulletin will average the 24 hours from Thursday afternoon, including the still night. Check the air at your nearest station, hour by hour, on the HT Pollution Dashboard at hindustantimes.com/breath
