The thick fog that has enveloped the city right at the beginning of December this year makes it look like peak winter.
Scientists said the early onset of fog this year was a warning that December this year might be the foggiest in the last five years.
Weather experts said the wind speed in the first week of December had been lower than that reported during the corresponding period in the last five years.
An assessment by the climatology department of the International Climate Brigade, which has been working with IIT-Delhi in a pollution research, shows that the average wind speed in the first week of December has been negligible.
“The problem is that the day time wind speeds have been low this time. Generally, the average wind speeds at night during winter is around 1 to 3kmph. This picks up during the day and reaches around 10 to 12kmph. This helps disperse pollution particles and clear the fog. This year, however, the day time wind speed has been low,” said Dr Satish Kumar Yadav, research head at International Climate Brigade.
Between December 1 and 7 this year, the average wind speed during the day has been around 5kmph, which is not strong enough to disperse fog, causing a drop in visibility during early morning and late night hours. Experts say that a wind speed of at least 12kmph is required to blow fog and pollution particles away.
The visibility during this period has oscillated between 200-250 metres, during the early morning and late night hours. On Thursday morning, the visibility dropped to 150 metres at the airport, which led to flight delays and cancellations.
In 2015, the average daytime wind speeds during the first week of December was around 10kmph.
Shambhavi Shukla, a research associate with the Centre for Science and Environment (CSE), said the pollution levels will only continue to get worse if such conditions persist. She said that with the temperature dipping and the fog getting denser, the pollution levels in December will also get more severe.
“We are surely headed for the foggiest December experienced in a long time. The temperature will go down further in the coming days and wind speed is also unlikely to increase. This will lead to the particulate matter and dust getting trapped near the surface and having an impact on visibility,” she said.
Explaining the conditions likely to persist in the coming days, Shukla said generally in the first week of December the wind speed is higher and subsequently as the month progresses the wind speed decreases. As the air gets heavier, it traps particles in the air and floats in the atmosphere.
The moisture in the air mixes with these particles causing a deadlier form of fog called smog, which was experienced in the city on the first week of November.
She said that unlike during the other time of the year when rain can bring down soaring pollution levels, in winters the showers only make the condition worse. The effects of this will also be seen in the early weeks of January.
“Winter showers are not enough to wash away the pollutants. In fact, it lets the dust from the ground escape and remain suspended in the air, making the air quality worse,” Shukla said.
However, some weather experts said that it was too soon to predict that if this December would be the foggiest or not. If wind speed picks up and the temperature does not drop much, the situation will not get worse.
“Yes, the fog this year has been denser compared to previous years, but it is too early to say that this will make way for a foggy month. A lot of other factors also play a part in dispersion of fog and if those are favourable, we might have a regular December,” a weather expert said.