New Delhi, Nov. 13 -- Delhi's air turned sharply more toxic on Thursday, with the concentration of PM2.5 across the city more than doubling in 24 hours to 244 micrograms per cubic metre by 6pm, four times the national standard, as a thick, smoky haze that usually thins by afternoon stayed trapped over the Capital in warm, damp and almost motionless air. Two things closed in on the city at once, an HT analysis of official pollution and weather data showed. The dome of air in which Delhi's pollution mixes and thins shrank to about 400 metres high on Thursday afternoon, from 1,400 metres two days earlier; and the north-westerly that carries smoke from the farm fires of Punjab and Haryana, slack since Diwali, was blowing above the city again at 34 km/h. The Central Pollution Control Board's air quality index for Delhi, a rolling average of the previous 24 hours, climbed from 313 at 2am to 366 by 6pm, gaining seven to eight points an hour through the afternoon. At 10pm, it reached 387 - 74 points higher than the 24 hours before it. In all, 21 of the 58 stations reporting were in the "severe" zone, above 400, by then, against one on Wednesday evening and none on Tuesday. The index for the day stood at 350, or "very poor", in CPCB's 4pm bulletin, up from 313 on Wednesday and 299 on Tuesday. The weather meant that the afternoon - which typically brings some relief from high pollution every day - did not see a cleaning that's routine. For instance, at 4pm on Wednesday, PM2.5, the fine particles that lodge deep in the lungs, had fallen to 92. It then crossed 200 by 1am --- pollutants typically gather at night --- and stayed near 220....