Why Delhi is yet to record a fog episode so far this year

It’s mid-December when usually Delhi records dense spells of fog. However, this year, the city has not even seen a major fog episode so far.

Explaining the reason behind this, experts said that parts of northern plains experienced two western disturbances (WD) so far, but none of them added sufficient moisture for the formation of fog. They added that high wind speeds during this period has also ensured that fog did not form in the manner it usually does.

India Meteorological Department (IMD) data for the Palam monitoring station shows that Delhi has recorded the lowest fog hours between December 1 and 14 in the last at least six years. In 2015, the city recorded 157 hours of fog, and in 2016, Delhi saw 126 hours of fog during this period. But this year, Delhi has seen seven hours of fog till December 14.

Last year, Delhi recorded 74 hours of fog over Palam which included a “very dense” fog spell on December 7, when visibility dropped to zero and operations were impacted at Delhi airport.

RK Jenamani, scientist at IMD, who analysed long-term fog data, said this is the first season since 2012 that even shallow to moderate fog has not been recorded so far. IMD classifies the intensity of fog in terms of visibility. When the visibility is between 1,000 and 500 metres, it is classified as “shallow”; between 500 and 200 metres is ”moderate”; below 200 metres is “dense”; and under 50 metres is ‘very dense’.

“By this time of the year, we generally get shallow to moderate fog, with a dense fog spell also occurring. After 2012, this is the first time we are seeing similar conditions where even CAT I and CAT II visibility procedures for shallow to moderate fog haven’t been deployed at the Delhi airport,” said Jenamani, who added that though low temperatures were ideal for fog formation, a stable wind pattern is needed for heavy fog spells.

“This is not limited just to Delhi, but the entire northern plains. Delhi will see another western disturbance from December 15-16, which may lead to some shallow to moderate fog in the morning, but from December 17 onwards, wind speed will again pick up and conditions will become unfavourable for fog,” Jenamani said.

This year, while four hours of fog was recorded on December 3 and 4, when visibility dropped below 1,000 metres, three fog hours were recorded on the night and morning of December 4 and 5.

Navdeep Dahiya, an amateur weatherman who runs ‘Live Weather of India’, an online platform that provides weather information, said the two western disturbances in the first week of December were both fairly weak, and could not add enough moisture. “The first western disturbance around December 2 and 3 was near Maharashtra, and by the time it reached Delhi, it fizzled out. The next western disturbance around December 5 and 6 did bring enough snowfall to the mountains, but Delhi only recorded a drizzle,” he said.

Dahiya said Delhi may stand a better chance of seeing fog after December 24. “Another western disturbance is expected this week, but it will again be weak... A better chance of good fog is expected only after December 24, when a stronger western disturbance might influence weather,” he added.

An official at the Delhi airport confirmed operations have been smooth so far. “There have been no delays or any impact on operations so far due to fog,” he said.

Dipankar Saha, former head of Central Pollution Control Board’s air laboratory, said Delhi will have to wait for ideal meteorological conditions. “If the difference between ambient temperature and dew point, which means the point the air is saturated with moisture, is sufficient, one will see fog. You also need clear skies and calm winds for moisture in the air,” he said.