Dense fog envelops Delhi, air quality nears ‘severe’

Experts attributed the fog to calm winds, which also led to a sharp fall in the air quality, to the brink of “severe” level

New Delhi

Low visibility at Tilak Marg station on Friday morning. (RAJ K RAJ /HT PHOTO)

The Capital was in the grip of “very dense” fog early Friday, with visibility of zero around 4.30am at the Palam observatory improving to only around 100 metres at 9am and the Safdarjung observatory recording visibility of 50 metres around 5.30am, which improved to 100 metres by 8.30am, according to the India Meteorological Department (IMD). Experts attributed the fog to calm winds, which also led to a sharp fall in the air quality, to the brink of “severe” level as the city clocked an air quality index of 397 at 4pm, according to the Central Pollution Control Board (CPCB).

Fog in the morning led to a delay of over 100 flights, but there was no cancellation, airport authorities said. IMD issued a yellow alert for Saturday, warning of “moderate” to “dense” fog and light rainfall.

“Calm winds prevailed over Delhi in the early hours of the day, leading to a very dense to dense fog. A moderate fog is likely to be observed in most places on Saturday morning as well, with dense fog in isolated places,” an IMD official said.

IMD classifies it as “shallow” fog when visibility is 500 metres to 1,000 metres, “moderate” fog when visibility is 200 metres to 500 metres, “dense” when visibility ranges from 50 metres to 200 metres and “very dense” when visibility falls below 50 metres.

Mercury falls

The minimum temperature in Delhi on Friday was 6 degrees Celsius (°C), which was around a degree below normal and over a degree higher than the 4.8°C recorded a day before.

The maximum temperature, however, recorded a sharp dip on the day, at 17.6°C, which was 1.4°C below normal and down from 21.2°C recorded a day before.

Mahesh Palawat, vice president at Skymet meteorology, said, “Cloud cover and the blanket of fog is the primary reason for the dip in the maximum temperature. The minimum also saw a rise. When there is a layer of cloud or fog, the heat cannot escape at night and vice versa during the day.”

Other stations recorded similar temperatures on the day. The Palam station recorded a maximum of 17°C and minimum of 7.6°C, Lodhi Road station 16.1°C and 6.4°C, and Ridge and Ayanagar stations 17.3°C and respective minimums of 7.5°C and 7°C.

IMD has forecast light rain, accompanied by thunderstorms, on Saturday. “Very light rain or drizzle is expected to continue on Sunday morning as well. This is due to the interaction of a western disturbance and easterly winds,” the IMD official said.

Air worsens

The 24-hour average air quality leading until 4pm on Friday was 397 (“very poor”), clocking a steady rise from AQIs of 357 (“very poor”) and 297 (“poor”) over the past two days.

As the air quality index on Thursday worsened to the “very poor” level, the Commission for Air Quality Management (CAQM) in NCR invoked Stage 3 of the Graded Response Action Plan (Grap) to curb pollution. Stage 3 was last invoked on January 3, but was revoked on January 5 as air quality improved.

Forecasts by the Centre’s Early Warning System for Delhi (EWS) have predicted Delhi’s AQI to remain in the “very poor” category until Saturday, before improving to the “poor” range on Sunday and Monday, depending on light rain in the city.

“The outlook for subsequent six days is that the air quality is likely to be in poor to very poor category,” the EWS bulletin said on Friday evening.