India, Dec. 24 -- A double whammy hit the national capital on Tuesday as air quality deteriorated into the "severe" category for the fourth time this month and then combined with dense fog to reduce visibility, impacting various travel operations.

Winds calming overnight caused the air quality to plummet, experts said. Delhi's 24-hour average air quality Index (AQI) stood at 412 at 4 pm on Tuesday - up from a reading of 373 at the same time on Monday.

However, an increase in surface wind speed - up to 20 km/hr - during the day led to a marginal improvement, and by 8 pm, the AQI was back in the 'very poor' (398) category.

This is the fourth "severe" air day this month and the seventh overall this year. Last year, in December, Delhi reco...