Delhi's smog came from farm fires lit after NASA's satellite passed

Delhi’s air quality deteriorated sharply on November 1. The AQI rose from 117 on Saturday to 261 on Sunday. By Monday morning, the hourly reading exceeded 300, the threshold for “very poor” air. The 4pm bulletin reported 324, the season’s first such reading.

The timing pointed to crop burning in Punjab and Haryana. Delhi’s pollution did not clear during the afternoon, unlike on cleaner October nights. PM2.5 nearly doubled around midday on October 31 and stayed high. Fine particles also formed a larger share of the pollution, consistent with smoke rather than road dust.

INSAT-3DS detected evening fires rising from 17 on October 29 to 90 on November 1. NASA counted 171 fires that day, but its afternoon pass missed many later fires. The article therefore links the pollution surge to regional burning, carried into Delhi under stagnant conditions.

PM2.5 means fine particulate matter small enough to penetrate deeply into the lungs. The article identifies it as the particle type that does the most harm because it reaches deep inside the respiratory system. It contrasts these particles with coarser material, such as road and construction dust.

The November 1 pollution was especially rich in PM2.5. Fine particles made up nearly six-tenths of all particulate matter by weight that night. On the October comparison nights, they made up just over four-tenths. This pattern supports the finding that smoke contributed heavily to Delhi’s dirty air.

The article does not describe specific diseases or medical outcomes. It does show why PM2.5 is an important measure: it tracks the most penetrating particles, and India’s daily limit is 60 micrograms per cubic metre. November 1’s average was far above that limit.

On the night of November 1, Delhi’s PM2.5 averaged 194 micrograms per cubic metre between 10pm and 6am. India’s daily PM2.5 limit is 60 micrograms per cubic metre. The measured level was therefore more than three times the national limit, showing how severe the episode became.

The comparison with earlier October nights is striking. PM2.5 averaged 68 on October 18 and 127 on October 28. Thus, November 1 was about 2.9 times October 18’s level and roughly 1.5 times October 28’s level. The article describes the same weather as occurring on those nights.

The difference was not simply a weather effect. October 18 and 28 had equally poor, or worse, conditions for trapping pollution. On November 1, however, pollution stayed high through the afternoon. That pattern indicated additional dirty air arriving from outside Delhi, including smoke from crop fires.

Weather can trap pollution near the ground, but it does not determine the pollution amount by itself. Delhi had little wind and strong temperature inversions before November 1. Yet October 18 and 28 had conditions that were just as bad, or worse, and their air was cleaner. This made an outside pollution source the key difference.

Normally, sunlight warms the ground during the day. Air then churns, pollution spreads upward, and street-level readings fall. PM2.5 fell by nearly two-thirds on October 28 and more than half on October 18. On November 1, it did not fall; afternoon levels were slightly higher than morning levels.

That persistent pollution indicated dirty air above the city or moving into it. PM2.5 nearly doubled around midday on October 31 and stayed high into Monday morning. Rising evening fires in Punjab and Haryana supplied the extra smoke while Delhi’s still weather trapped it.

The article’s evidence does not describe a north-west wind pattern. Instead, it says readings stayed high because the air higher up was dirty or because dirty air was blowing in. It concludes that the extra pollution on November 1 came from outside Delhi. The reported weather near the city was otherwise calm.

The mechanism is regional transport. Smoke released over Punjab and Haryana can move downwind toward Delhi. Once there, weak winds and a temperature inversion can prevent mixing and dilution near the ground. This allows fine particles to accumulate, especially overnight. The article connects this mechanism with crop-fire timing and Delhi’s persistent readings.

The strongest evidence was the daytime pattern. Delhi’s PM2.5 normally fell as sunlight mixed the air, but it did not fall on November 1. The article therefore supports outside smoke entering Delhi, while leaving the exact wind direction unstated.

NASA’s commonly quoted fire count comes from a satellite that passes over Punjab once a day, at about 1:30pm. A fire that starts later may not be visible during that pass. The count can therefore understate burning, especially when farmers increasingly light fires in the evening.

The article gives a clear contrast. More than 90% of large Punjab fires in 2024 and 2025 were lit after 3pm, compared with only 3% in 2021. India’s INSAT-3DS geostationary satellite scans the area throughout the day, so it captures the timing better, although its lower detail detects only larger or more intense fires.

Last season, 99% of fires INSAT-3DS spotted in Punjab and Haryana were burning after 3pm. This season, its count rose from 17 on October 29 to 90 on November 1. NASA counted 171 that day, but still missed many later fires.

The article does not state why farmers burn leftover crop stubble, so the reason cannot be established from the provided text. It focuses instead on when burning happens and how the resulting smoke affects Delhi. The fires occurred mainly in Punjab and Haryana, with a sharp rise in evening detections before November 1.

Burning stubble produces smoke. The article says smoke is mostly made of fine particles, unlike road and construction dust, which is coarse. On November 1, fine particles made up nearly six-tenths of all particulate matter by weight. That composition helped distinguish crop smoke from ordinary dust pollution.

The smoke mattered because it came from outside Delhi and arrived while local weather was stagnant. PM2.5 nearly doubled around midday on October 31 and remained high. Thus, the documented chain is crop burning, fine-particle smoke, regional transport, and pollution trapped over Delhi.