‘Dense fog’ blankets Delhi; visibility dips to zero at Palam

A “dense fog” blanketed Delhi for the first time this season on Wednesday dipping visibility to zero at the Delhi airport even as calm winds prevailed across the region and continued to prevent the dispersion of pollutants deteriorating the air quality to the higher end of the “very poor” category.

An average Air Quality Index (AQI) of 361 (very poor) was recorded at 8am compared to 334 (very poor) at 4pm on Tuesday. Only Ayanagar out of the 36 monitoring stations recorded a severe average AQI of 417 at 8am. Lodhi Road (268) and Aurobindo Marg (275) recorded “poor” air quality. Overall, this was the 15th consecutive day of “very poor” air quality.

The Central Pollution Control Board (CPCB) classifies AQI between 0-50 as “good”, 51 and 100 as “satisfactory”, 101 and 200 as “moderate”, 201 and 300 as “poor”, 301 and 400 as “very poor”, and over 400 as “severe”.

India Meteorological Department (IMD) officials said the visibility at the Delhi airport (Palam) was 100 metres at 6am before dipping to zero at 8:30am. “This is the first dense fog for Delhi this season. Visibility was zero at the Delhi airport (Palam) at 8:30am and the runway visual range was 125 metres. Similar fog was reported across northwest India with zero visibility reported at Amritsar and Hindon airports, 150 metres at Deoghar airport, and around 200 metres at Chandigarh and Gorakhpur,” said an IMD official.

IMD classifies visibility between 500 and 1,000 metres as “shallow”, 200 and 500 metres as “moderate”, and 50 and 200 metres as “dense”. Visibility lower than 50 metres is classified as “very dense” fog. The lowest visibility reported in Delhi this season was 700 metres at Safdarjung on Monday.

The IMD said mostly fog prevailed across Delhi over the past two weeks. It added it was fog and not smog in the early hours of the day. The IMD calls it fog if the relative humidity is over 75% and smog if it is over 75%, with visibility less than 1,000 metres in both cases.