Spells of dense fog this winter highest in 5 yrs

This was the sixth winter season in a row when the city failed to log the normal long-term average mark of 99 hours of dense fog hours

New Delhi The Capital recorded 89 hours of “dense fog” this winter season, making it the most intense spell of fog witnessed by Delhi since the 92 hours logged in 2017-18, India Meteorological Department (IMD) data showed.

Lodhi Garden on in New Delhi on Sunday. (Sanchit Khanna/HT Photoimes)

The IMD defines dense fog as when visibility drops to lower than 200 metres for a full hour.

However, this was the sixth winter season in a row when the city failed to log the normal long-term average mark of 99 hours of dense fog hours -- the last time the city breached the normal was in 2016-17, when it recorded 125 hours of dense fog.

An IMD scientist said this winter season, Delhi recorded its first dense fog spell only in a limited window of December 19 to January 11. “We saw very little dense fog in December as it started fairly late. Minimum temperatures were high and there was not enough moisture till the third week of the month,” the scientist said.

Noting that January’s spell of dense fog also came fairly early, and dense fog hours dropped considerably after January 12, the scientist added, “Minimum temperatures once again started rising from January 12, with active western disturbances seen during that period, which also led to strong winds. Dense fog does not form when wind speeds are high or there are cloudy skies.”

This season’s most intense dense fog spell came on January 9, when visibility was below 200 metres for around 10 hours between 1am and 11am, leading to over 200 flights getting delayed at Delhi’s Indira Gandhi International airport, while five flights were diverted to the nearby Jaipur airport. A day prior to that, the city logged dense fog for nine hours between 2:30am and 11:30 am, IMD data showed.

Kuldeep Srivastava from IMD’s Regional Weather Forecasting Centre (RWFC), which looks at Delhi’s weather, said for fog to be dense, three meteorological conditions need to be in sync – calm winds, high moisture content (over 90% relative humidity) and largely clear skies. He said these conditions were missing for a majority of December, with lower humidity levels and strong winds in the region.

“From December 19 onwards, the conditions became more favourable, with fog gradually stretching across the Indo-Gangetic Plains, but over the last few years, we have seen these conditions have not been in sync at the same time and so the intensity of fog has not been what it normally should be,” he said.

“Sometimes, high pollution levels can negatively impact fog formation. At other times, visibility could be low mainly because of high pollution which is then not fog, but smog,” he added.