A pollution link to why Delhi gets heavy winter fog

A study by Indian Institute of Tropical Meteorology and India Meteorological Department shows that the intensity of fog during the winter season has been increasing over Delhi due to the high pollution load.

Delhi and many other parts of northwest India experience severe dense fog almost every winter disrupting air and train schedules and making road passengers more susceptible to accidents.

People out on a foggy morning at Moti Bagh in New Delhi, on 28 December.(HT PHOTO)

But why is NW India, more specifically the Indo-Gangetic Plain region susceptible to dense fog of this scale?

Meteorologists explain its mainly due to four reasons-- low winds, low temperatures, availability of moisture and pollution particles which act as surface for condensation. Delhi’s air quality index on Monday morning was 448 in severe category.

Fog is basically suspended water droplets or ice crystals in air very close to the surface.

A study by scientists of Indian Institute of Tropical Meteorology and India Meteorological Department published in 2017 in the journal of Aerosol and Air Quality Research (AAQR) the frequency and intensity of fog episodes during the winter season has been increasing during the past decade (2006 to 2016)over the megacity of Delhi due to the high pollution load. “The role of atmospheric aerosols is very important in the life cycle of fog in the urban areas,” the study said.

Mass concentration of Black carbon (BC)-- a component of particulate matter increased before and during the initial phase of a dense fog period. The study found decrease in temperature and increase in relative humidity (RH) played a major role in sustaining the dense fog despite the reduction in aerosol load thereafter.

Meteorologists have been noticing the phenomenon of extremely low day temperatures or cold spells accompanied by dense fog or low cloud cover over NW India more often since 1997. Delhi has been experiencing a cold spell for 16 days now.

“In 1976 there was a cold spell for six days. But after that the phenomenon has become more common in the late 90’s. If fog or low cloud hours increase obviously less solar energy is received and hence days are cold,” said RK Jenamani, senior scientist at National Weather Forecasting Centre.

Jenamani published a study in the Current Science journal in 2007 which said that there has been a fall in average maximum temperature by 2–3 degree C over 2 stations in the peak winter of January since 1989 with increase in average fog (all intensities of fog) hours per day by 8 h during the same period.

This study found that air pollution accentuates fog which in turn plays a role in reducing day temperatures because fog blocks sunlight. “The peak fog seasons were seen in the 1996 to 2007 period. Moisture and air pollution are highly linked. The life period of suspension of air pollutants is prolonged when there is moisture in the air. Low clouds which we are seeing over a very large area and very long period is nothing but fog at slightly higher level from the surface,” said Jenamani.

“When moisture is high pollution also goes up because particles become heavy and do aid in formation of fog or low clouds. But this needs to be studied further. The previous western disturbance on December 12 and 13 was so strong that it brought a lot of rain in December and caused moisture incursion helping create a severe cold spell and fog” Shrivastava said.

“In the past few days we have seen low clouds descend to surface and cause dense fog and then rise up again in a few hours. Availability of moisture and low temperatures combined to create the low cloud cover.” said Sachin Ghude, senior scientist at Indian Institute of Tropical Meteorology.

IMD describes fog as a phenomenon of small droplets suspended in air and the visibility is one kilometer or less.

Fog condensation usually occurs on particles floating in the air.