Weather, saviour against Delhi pollution until now, turns rogue
With the drop in wind speed in Delhi, as temperature dropped in the early hours of Wednesday, the two factors combined to create the noxious smog
The Capital’s relative reprieve from post-Diwali smog this year came to an abrupt end on Wednesday, with two critical weather patterns — a delayed winter and surface winds — suddenly turning against the city.
As dawn broke on Wednesday, Delhi’s residents found themselves enveloped in a noxious haze, the air thick and acrid. This one-two punch of cooling temperatures and calm winds left residents choking on the season’s first “severe” air day, as AQI levels surged past 400, plunging into hazardous territory.
Late Tuesday night, the AQI in Delhi was hovering lower end of the “very poor” zone – the 24-hour average was around 314 at around 10pm. But as the night wore on, winds eased and temperatures dropped, leading pollutants to settle over the city like a dark grey veil.
It started with a drop in wind speed, leading to pollutants accumulating in the night sky. And as temperature dropped in the early hours of Wednesday, the two factors combined to create the noxious smog.
The smog also kicked off, for the first time this season, the cyclical phenomenon known as “inversion” – where pollutants get trapped close to the surface due to low temperature.
Inversion takes place when the surface air is cooler than the air at higher altitudes – the inverse of what usually happens. For this, a combination of both calm winds and low temperature is needed, leading to the mixing height – an invisible layer where pollutants interact -- dipping and coming closer to the surface. The lower the mixing height, lesser the space for pollutants to move freely and escape. This created a “lid-like” layer over the Capital.
Delhi was recording “above normal” temperature for consistently for the last few days. The maximum on Wednesday, however, went below normal for the first time this season, according to IMD data.
The high temperature and the slow winds had so far been key to helping the city avoid the brunt of post-Diwali pollution. For example, the bursting of firecrackers generally turns Delhi’s air severe in the days after the festival. But it did not happen this year because a warm sun and strong winds lifted the smoke from the firecrackers and carried it away.
The maximum temperature on Wednesday was 27.8°C, one degree below the normal and a massive five degrees below Tuesday’s 32.8°C. The minimum was however, still three degrees above normal for this time of the season and was recorded at 17°C, as compared to 17.9°C a day before.
“The very dense layer of fog in the morning led to a decrease in the maximum as the Sun’s heat could not reach the earth’s surface properly,” said a senior India Meteorological Department (IMD) official.
IMD has issued an orange alert for Thursday, warning people of a dense to very dense fog in the early hours of the day.
The fog, caused by an interaction between warm and humid easterlies and cold and dry westerlies, led to a decreased visibility in Delhi and a spike in the air pollution levels.
According to data provided by Central Pollution Control Board (CPCB), the first severe AQI in 2023 was recorded on November 3 and on November 1 in 2022. CPCB began calculating AQI only from April 2015 onwards and according to the data available, AQI deteriorated to severe in the last week of October or first week of November in the previous years, dating back to 2015.
The 24-hour average air quality index (AQI) was recorded at 418 (severe) at 4pm, according to the Central Pollution Control Board’s (CPCB) national bulletin. This was a drastic deterioration from 334 (very poor) AQI, recorded a day before.
The Air Quality Early Warning System (AQEWS) for Delhi, however, has forecast the air quality to go back to “very poor” by Friday, owing to surface winds expected in the next few days.
To be sure, AQEWS was unable to predict on Tuesday that the air would turn “severe” on Wednesday.
Anumita Roychoudhury, an air pollution expert and executive director at Centre for Science and Environment, said, “As the weather turns cold, the air quality has a tendency to degrade. However, if the surface wind speeds pick up again, as has been forecast, then the natural ventilation will allow dispersal of pollutants and keep pollution levels relatively lower.”
IMD forecasts suggest that there is a possibility of dense to very dense fog conditions in the early hours of Thursday, which might translate to a smog by the later half of the day. A moderate to dense fog is likely to form on Friday as well.
According to an IMD official, the calm wind conditions that prevailed over the city and led to fog conditions might pick up from the following weekend. “A shallow to moderate fog or mist might be observed over the weekend,” the official added.
Mahesh Palawat, vice president at Skymet, said, “A western disturbance is approaching the western Himalayas and is likely to cause fresh rain or snowfall in the next two to three days. Once the wind from the Himalayas starts impacting Delhi and the neighbouring areas, we will see a gradual decline in temperature. We can call this the commencement of winter.”
IMD has forecast the maximum to stay between 28°C and 29°C till November 19. The minimum is expected to drop to 14°C by beginning of next week.
IMD officials, however, said, “We are currently in the post-monsoon season which spans over October, November and December. Winter season starts from December and runs through February.”