Why is Delhi not recording much fog this winter season?

Not only is the lack of “dense fog” hours strange, but Delhi has also not recorded enough shallow to moderate fog hours

New Delhi: Delhi may have seen peak winter weather with the minimum drop to a low of 3.2 degrees Celsius at Safdarjung on December 20, making it the lowest of the season, but a key element of the winter — dense fog — has been notably missing. Delhi usually tends to see its first “dense fog” spell by the first or second week of December. However, the city only recorded one dense fog hour all December. In January, Delhi was yet to record a single hour of dense fog until Tuesday morning, when the capital recorded only its second dense fog day of the season. Visibility fell to 50 metres at the Indira Gandhi International (IGI) airport, which is yet to face any cancellations or flight diversions this winter. Similarly, while trains have faced some delays, no train cancellation has occurred this season on account of dense fog — a rarity for this time of the year.

CAT-I procedures are initiated at Delhi’s Indira Gandhi International airport in “shallow” fog, while CAT-II procedures are initiated in “moderate” fog(PTI)

Experts say weak western disturbances could partly be blamed for this unique period of very little fog activity, each of which has failed to add “sufficient” moisture, which leads to fog formation. The only day Delhi recorded dense fog in December was on December 29, when visibility dropped below 100 metres at Palam between 9 and 10 am. Delhi had received 3.4mm of rainfall on December 27 from a western disturbance, while another 3.6mm was recorded the next day, which met officials say helped in dense fog formation.

December had also seen two back-to-back western disturbances at the start of December, but neither was able to bring sufficient moisture or rain. Similarly, this month, Delhi’s only dense fog spell on Tuesday morning came on the back of two adequate spells of rain on Saturday and Sunday, adding over 50mm of rainfall to Delhi.

RK Jenamani, a scientist at the India Meteorological Department (IMD), says long-term data shows such a long spell of very little dense fog is rare, with Delhi recording a similar spell last seen in 2012. However, Jenamani says not only is the lack of “dense fog” hours strange, but Delhi has also not recorded enough shallow to moderate fog hours.

The IMD classifies visibility between 1000 and 500 metres as “shallow”, between 500 and 200 as “moderate” fog, below 200 metres as “dense” fog and visibility below 50 metres is classified as “very dense” fog.

“In December, only one dense fog hour was recorded and prior to Tuesday, CAT-III procedures were yet to be initiated at the Delhi airport. This shows how important a strong western disturbance is in winter,” says Jenamani.

CAT-I procedures are initiated at Delhi’s Indira Gandhi International airport in “shallow” fog, while CAT-II procedures are initiated in “moderate” fog. When visibility drops below 200 metres, CAT-III and CAT-IIIB procedures are initiated, allowing only a limited number of flights or pilots to land. For this, the flight, as well as the airline, needs to be trained to land in dense to very dense fog.

IMD data from the Palam Observatory, which is considered for the Delhi airport, shows there were seven days and 28 hours of “dense” fog in December of last year. In January, 10 days of dense fog, with 43 “dense” fog hours were recorded. This year, Delhi has only recorded two dense fog days and a total of five hours of dense fog, four of which came on Tuesday morning.

Wind speed is another key factor influencing dense fog formation, experts say, with Delhi recording fairly good wind speed in December. Navdeep Dahiya, an amateur weatherman, who runs Live Weather of India, says conditions remain ideal this week for dense fog, something that was missing earlier.

“Last month, we were seeing weak western disturbances which were not adding enough moisture and wind speed was also remaining high soon after the western disturbance departed. During high winds, fog is unable to form or stay over the surface. In comparison, we now have sufficient moisture from this weekend’s rain and wind speed is below 10 km/hr, leading to the dense fog spell in Delhi on Tuesday,” Dahiya said.

Comparing data from last year, Dahiya says the northern plains also witnessed a strong anti-cyclonic circulation over a long period, which allowed fog to settle in the northern plains. This year, that has been missing, with frequent western disturbances leading to “instability” in the weather system. “If you see back-to-back western disturbance and not enough moisture is getting added, there will be high winds and little moisture for the fog to form”, he says.

M. Mohapatra, Director General, IMD, says while fog hours had been low in Delhi and parts of the northern plain, it did not form too much of a concern just yet. “If this trend is seen across January and February, then certainly it will be a very unique winter, however, such an occurrence is possible over a month’s duration where weather factors may be unfavourable.”

An official from the Delhi airport said no flights had been cancelled or diverted yet this winter season on account of the weather, with a handful of flights facing some delays on Tuesday. “Operations have been smooth this winter season. Fog is yet to play much of a role so far this season,” the official said.

Similarly, Kultar Singh, PRO, Northern Railway says the impact of fog has been negligible this winter. “We have seen some impact in parts of Uttar Pradesh and Punjab, but there has been no impact in Delhi. We are yet to report any major delays or cancellations on account of fog, unlikely previous years,” he said.

Dipankar Saha, former head of Central Pollution Control Board's air laboratory says Delhi is likely to witness ideal conditions for fog formation this week, which includes a sufficient enough difference between the maximum and minimum and low wind speed. "Clear skies and high wind speed make it difficult for moisture to stay in the air. This trend has been reversed after this weekend’s rainy spell and we could see a return of fog in the northern plains," he says.