'Moderate fog' in Delhi as mercury dips below 10 deg C, air quality 'very poor'

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Delhi's minimum temperature dropped to 9.5 degrees Celsius and a blanket of fog covered the national capital, affecting visibility on Tuesday morning. On Monday, “very dense” fog had lowered visibility in some areas to “zero” metres.

The Safdarjung and Palam weather stations recorded a visibility of 300 metres due to “moderate” fog at 8:30 am, an official of the India Meteorological Department (IMD) said.

The visibility should be around 800 metres at the airport for flights to take off, said Kuldeep Srivastava, the head of the IMD's regional forecasting centre.

According to the IMD, “very dense” fog is when visibility is between 0 and 50 metres, 51 and 200 is “dense”, 201 and 500 “moderate”, and 501 and 1,000 “shallow”.

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The mercury is likely to dip further with the wind direction changing to westerly-northwesterly. These icy cold winds blow from snow-laden western Himalayas towards the plains.

Delhi's air quality was recorded in the upper end of the “very poor” category on Tuesday. The city's air quality index (AQI) was 393 at 9 am on Tuesday.

The 24-hour average was 400 on Monday, 389 on Sunday and 404 on Saturday.

Air pollution in the neighbouring cities of Ghaziabad (AQI 436), Greater Noida (415) and Noida (414) remained in the “severe” zone for the third consecutive day.

An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".

The IMD said the maximum wind speed will be around 12 kmph on Tuesday.

Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.