New Delhi, Nov. 3 -- The primary driver pushing Delhi's air quality into the "very poor" category on Monday was smoke from evening farm fires in Punjab and Haryana, according to an analysis of weather, air-quality, and satellite data by HT.

Delhi's Air Quality Index (AQI) surged from 117 on Saturday to 261 on Sunday. By Monday's 4 PM bulletin, the index hit 324-crossing the 300-mark threshold for "very poor" air for the first time this season, a level not recorded since February 11, HT reported.

While stagnant weather conditions played a role, HT's data comparison indicated that meteorological factors alone do not explain the sudden deterioration in Delhi's air quality.

Measures including afternoon wind clearance, HT found that dawn in...