Most of the fires were lit in the evening, after the NASA satellite whose count is the standard measure of stubble burning had passed over the region.
Smoke from farm fires in Punjab and Haryana drove the sharp rise in Delhi’s pollution on Sunday, which pushed the air into the ” very poor ” range on Monday, an HT analysis of air-quality, weather and satellite data has found.
The weather in the run-up was poor, with little wind and a still night. The inversion is a lid of warm air that stops air near the ground from mixing upwards. Farmers no longer burn at that hour.
Because they reach deep into the lungs, the clearest measure is PM2.5, the fine particles that do the most harm. Delhi’s air quality index, which runs from 0 to 500, rose from 117 on Saturday to 261 on Sunday. Monday’s 4pm bulletin put the index at 324, the season’s first very poor reading. The last very poor reading this year was on February 11. But Delhi had the same weather on October 18 and 28, and the air on those nights was far cleaner. India’s limit for PM2.5, averaged over a day, is 60 micrograms per cubic metre of air. On the night of November 1, from 10pm to 6am on Monday, it averaged 194, more than three times that limit. On the October nights it averaged 68 and 127. HT compared November 1 with every night this season on three measures from the India Meteorological Department’s weather balloon and the Safdarjung airfield: how well the afternoon wind cleared the air, how strong the dawn inversion was, and how still the night was. The count most often quoted for stubble burning comes from a NASA satellite that passes over Punjab once a day, at about 1.30pm. More than 90% of large fires in Punjab in 2024 and 2025 were lit after 3pm, against 3% in 2021, according to the Stubble Burning Status Report 2025 by iFOREST, a New Delhi research group.
By Monday morning the Central Pollution Control Board’s (CPCB) hourly reading had crossed 300, the threshold for “very poor” air.
HT also used wind data from two global weather models to trace where the air reaching Delhi had been over the previous three days. On November 1, it had spent most of that time over Punjab and Haryana, passing over dozens of fires that the satellites had spotted in the day before. On October 28, it came in from the east, away from the fire belt, and passed few fires. On October 18, it crossed Punjab, but before the evening burning had begun, and the city stayed comparatively clean.
