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Season’s first fog blinds Delhi for two hours

PUBLISHED ON NOV 30, 2019 08:54 PM IST

New Delhi

Delhi witnessed the season’s first fog on Saturday, with the visibility dropping to zero for nearly two hours in the morning. The low wind speed also deteriorated the overall air quality, pushing it to the ‘moderate’ category.

However, no flights were delayed, rescheduled or diverted due to Saturday morning’s weather conditions, airport officials said.

Scientists at India Meteorological Department (IMD) said that even though on two days in early November there were spells of ‘shallow fog’ in parts of Delhi, this was the first episode of ‘dense fog’ where visibility levels touched zero metres for nearly an hour in the early hours of Saturday.

On Saturday, the maximum temperature was logged at 22.8 degree Celsius, three degrees below normal. The minimum temperature was 12 degree Celsius, which was two degree above the season’s normal. According to Central Pollution Control Board (CPCB) data, the overall average air quality index (AQI) of Delhi was 193, in the ‘moderate’ range. This was a dip from Friday’s 84, which was in the ‘satisfactory’ category.

Experts explained that on Saturday morning, between 6am and 7am, the visibility at Safdarjung and Palam observatories dropped to zero metres. The fog was mainly caused by the recent rainfall in Delhi and parts of North India last week, because of which temperatures fell and the moisture in the air increased, forming perfect condition for fog.

The low wind speed also led to low dispersion of fog and pollution particles.

Saturday’s fog — also known as ‘radiation fog’ — forms overnight as the air near the ground cools and stabilises. When this cooling causes the air to reach saturation point, fog forms. Fog will first form at or near the surface, thickening as the air continues to cool. The layer of fog will also deepen overnight as the air above the initial fog layer also cools. As this air cools, the fog will extend upward, eventually affecting visibility.

“The fog sequence happens usually after a western disturbance. Since the sky is clear, the moisture in the atmosphere along with the dip in temperature becomes a catalyst for fog,” said Kuldeep Srivastava, head of IMD’s regional forecasting centre.

Srivastava said that the wind speed on Saturday was also in the range of five to eight kilometres per hour, which trapped the fog and also did not allow the pollution to disperse.

“But things got better as sunlight hit the ground,” he added.

Another IMD scientist said that fog is forecasted for Sunday morning as well and in the coming week as the weather in the Capital continues to get colder.

“From November 4, the wind speed will also reduce, which is also likely to affect air quality a little bit. However, the deterioration will not be too much,” the senior scientist said.

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