New Delhi, Dec. 16 -- A dense sheet of toxic fog held the Capital in a chokehold on Monday, sending visibility plummeting to zero in swathes of the city, disrupting road and rail traffic, and causing over 800 flights to be delayed and another 228 cancelled as of 10pm, despite recent airport upgrades. The first 'dense to very dense' fog of the season blanketed a city already gasping for breath in air that remained in the severe category for a third consecutive day. The 24-hour average air quality index (AQI) stood at 427 at 4pm - marginally lower than Sunday's reading of 461, which was December's second worst air day ever. The noxious cocktail forced the Delhi government to shift schools from hybrid to online mode for students up to Class 5 but there was little respite for the Capital's 25 million residents caught between official apathy, poorly designed response systems, and a long winter in front of them. The India Meteorological Department (IMD) issued a yellow alert for Tuesday, forecasting moderate fog in the early hours of the day, with dense fog for isolated pockets. However, strong surface winds of 15-25 km/hour are forecast during the day, which might help with dispersion of pollutants. The grey skies over Delhi bore a grim reflection of the weekend, when surface pollutants stayed trapped under slow winds, dipping temperatures, and an "inversion" layer, even as a weak sun failed to disperse them. Residents continue to complain of laboured breathing and burning eyes - which have now become all too normal - and the weather conditions served as a reminder of collapsing systems that fail to mitigate the yearly health crisis that descends on the Capital. This was the sixth straight severe air day this year. Three such days came between November 11-13. The IMD classifies fog as shallow when visibility is between 500-1000 metres, moderate when visibility is between 200-500 metres, as dense when it is between 50 and 200 metres and as very dense when visibility dips below 50 metres. The low visibility threw operations at the Indira Gandhi International Airport - which has barely recovered from the scheduling meltdown of IndiGo that grounded tens of thousands of passengers earlier this month - into disarray. The airport saw 800 flights delayed through the day, 228 cancelled and five diverted. Northern Railways reported over 90 trains delayed by between 30 minutes to five hours....