Delhi: Strong winds help reduce pollution

Tuesday’s AQI was an improvement on Monday’s reading of 218 (poor) and the best air day in December seen since December 14, 2020, when the AQI was 160 (moderate).

Strong northwesterly winds led to a drop in pollution in Delhi on Tuesday, with the Capital recording an Air Quality Index (AQI) reading of 177 (moderate), as per the daily bulletin released by the Central Pollution Control Board (CPCB) at 4pm -- the cleanest air logged by the city in the month of December in two years.

Delhi recorded a minimum temperarture of 8.9°C on Tuesday, which is around normal for this time of the year. (Sanchit Khanna/HT Photo)

Tuesday’s AQI was an improvement on Monday’s reading of 218 (poor) and the best air day in December seen since December 14, 2020, when the AQI was 160 (moderate). Tuesday was also the first time in nearly two months that Delhi recorded a ‘moderate’ air day, with the last such day logged on October 15 (186).

The CPCB classifies an AQI between 0-50 as ‘good’, between 51 and 100 as ‘satisfactory’, between 101 and 200 as ‘moderate’, between 201 and 300 as ‘poor’, between 301 and 400 as ‘very poor’ and over 400 as ‘severe’.

“Following the departure of a western disturbance that was influencing north India until December 11, there has been an increase in wind speed, with cold northwesterly winds blowing towards Delhi-NCR. The wind speed on Tuesday was around 15-20 km/hr and this is likely to help improve the AQI, while also gradually bringing down the temperature,” said RK Jenamani, scientist at India Meteorological Department (IMD).

Experts also said that warmer-than-normal temperatures have led to a high mixing height – an invisible layer of the atmosphere which can trap pollutants – during the day, which allows pollutants to disperse easily. On Tuesday, Delhi recorded a maximum temperature of 26.8 degrees Celsius (°C) -- four notches above normal for this time of the season.

In terms of minimum temperature, Delhi recorded a low of 8.9°C, which is around normal for this time of the year. While the maximum is expected to dip to around 25°C degrees by Thursday, the minimum is expected to touch 6°C by then.

“While cold northwesterly winds are starting to blow once again, the last western disturbance seen in northern India was not very strong and so it did not bring enough moisture to Delhi, and the dip in temperature has also been gradual. The mixing height is fairly high at the moment and temperature is still above normal, allowing pollution to disperse under the influence of these strong winds,” said Mahesh Palawat, vice president at Skymet Weather Services, a private forecasting service.

Palawat said during a strong western disturbance, the moisture content rises significantly, leading to cloudy skies and fog, which in turn traps pollutants close to the surface.

Forecasts by the Early Warning System (EWS) for Delhi, a model under the ministry of earth sciences, shows the AQI is likely to be ‘moderate’ till December 15, before touching ‘poor’ once again on December 16.

“These relatively strong winds and better ventilation are likely to prevail from December 13 to 15. The air quality will improve further, but will remain in the ‘moderate’ category on Wednesday. It will deteriorate marginally on Thursday but will remain in the ‘moderate’ category on Thursday and it is likely to touch the ‘poor’ category on Friday,” EWS said.