Delhi sees cleaner air in first half of December, how change in wind pattern may have helped


The air quality in Delhi for the first two weeks of December this year has been the cleanest it has been for the first half of the month since at least 2016. The average AQI from December 1-16 this year was 295.

For the same period last year, it was a higher figure of 324, while it was 330 in 2020 and 312 in 2019. In 2016, this average was as high as 365.

So far this month, the city has recorded a single day when the AQI was in the ‘severe’ category, over 401.

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While 2021 and 2020 also saw only one such ‘severe’ air day each, in the first half of December 2019 and 2016, there were three such days.

The better air quality for the first half of this month has been on account of winds, according to Gufran Beig, founder project director, SAFAR (System of Air Quality and Weather Forecasting and Research).

“The trough line extends from below Rajasthan, towards Maharashtra and all of South India, and above it lies Bihar, Uttar Pradesh, Rajasthan. This is the line over which western disturbances get divided. Because of the disturbances in large scale circulation triggered by consecutive La Nina conditions, the wind speed was relatively higher above this trough line and was relatively calmer below the trough line. This could be why North India in general has been facing relatively better air quality. Surface winds have remained relatively higher throughout November and December so far over North India. La Nina conditions tend to change large scale circulation patterns,” Beig explained.

La Nina conditions refer to cooler temperatures of the ocean surface in the equatorial Pacific Ocean that brings about changes in atmospheric circulation and impacts weather.

A drop in temperature and slow winds, however, are likely to lead to a deterioration in air quality over the next three days, according to a forecast issued by the SAFAR forecasting system on Saturday. The AQI on Saturday had already deteriorated to 304 in the ‘very poor’ category, having worsened from 163 in the ‘moderate’ category on Wednesday, December 14.

On the worsening air quality, Beig said, “The next 10-15 days remains to be seen. The impact of the change in wind patterns has been taken over by a fall in temperature.”

The air quality in November this year also fared better than previous years.


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