How speed, direction of wind dictate quality of air we breathe
The speed and direction of winds — specifically the high, long-range transport winds — can either wash pollutants away and clean up the air.
This year, like clockwork, Delhi began choking on a toxic cocktail of pollutants in early November, as smoke from scorched paddy fields in Punjab wafted towards the Capital and blanketed its air. The city’s tryst with hazardous air pollution levels has become an annual affair, and even as emissions (local and external) play a key role in regulating Delhi’s air quality, one weather factor plays a key role in pushing the pollution needles one way or the other — winds.
The speed and direction of winds — specifically the high, long-range transport winds — can either wash pollutants away and clean up the air, or keep emissions trapped in Delhi and worsen the pollution.
Delhi’s 12 “severe” or “near severe” air days so far in November — when the air quality index (AQI) is over 390 — is an example of the latter, with calm, predominantly northwesterly winds bringing smoke from farm fires in Punjab and Haryana to the Capital and trapping pollutants locally.
However, on the flip side, the Capital has also seen the winds lend a helping hand. November 18 was the latest such instance, when a change in wind direction from northwesterly to easterly improved Delhi’s average AQI from 405 (severe) a day earlier to 319 (very poor).
Delhi also got relief from pollution on November 10, when overnight rains led to a massive drop in AQI, from 437 to 279. But the rain, too, was precipitated by a change in wind direction — from northwestly to easterly.
“This was Delhi’s cleanest spell of air this November, and though rains helped on the first day (November 10), the average AQI remained in the ‘poor’ zone for the next two days too, aided by an easterly wind direction and moderate wind speed during the day,” said Mahesh Palawat, vice-president of private weather forecaster Skymet.
The day after Diwali, November 13, Delhi’s air began to deteriorate again as firecracker emissions added pollutants to the atmosphere. This coincided with a return of northwesterly winds, which kept AQI on the brink of severe on November 14 (397) and 15 (398), before giving Delhi back-to-back severe air days on November 16 (419) and 17 (405).
“AQI improved only over the last two days (starting November 18), when a return of easterly winds aided in improving the air quality locally,” Palawat said.
Northwesterly wind coincides with a high farm fire count at this time of the year, with the peak fire count occurring in the first or second week of November. The highest single-day fire count in Punjab this year has been 3,230 fires, which was recorded on November 5. Last year, a peak of 3,916 fires came on November 11. This coincides with unfavourable meteorological conditions over Delhi.
Experts said another characteristic of November is calm wind, particularly at night. This, they said, happens in November as it is a transitional month, following the withdrawal of the southwest monsoon in October.
“By mid-October, we see withdrawal of the monsoon from the entire country and soon after, we see a change in wind direction, from easterly to northwesterly. The atmosphere also becomes stable and strong northwesterly winds are seen more towards late December, when winter has established completely,” said Madhavan Rajeevan, former secretary at the Union ministry of earth sciences (MoES).
“In November, these winds are much slower and can become almost calm, until the transition to winter is complete. A northwesterly wind direction coincides with stubble burning too,” he said, adding that calm winds for a sustained period is what pushes Delhi’s into the severe zone. “On days where we see severe air, the wind direction is almost always northwesterly and the speed is almost calm,” he added.
Slow winds also traps pollutants close to the surface and subsequently causes inversion — a phenomenon when pollutants create almost a lid over the city and lead to poor visibility throughout the day.
Kuldeep Srivastava, scientist at IMD said Delhi’s pollution levels will likely improve on Tuesday, when wind speeds are expected to touch 15 kmph during the day. “Though the wind direction will be northwesterly, the speed will be fairly strong – that could clear up visibility,” he said, adding that the switch to easterly winds on November 18 was the impetus Delhi’s air needed.
Meanwhile, data by the Indian Agricultural Research Institute (IARI) shows that farm fires are on a decline in Haryana, with the northern state logging 634 stubble burning incidents on Monday, down from 740 and 637 on the previous two days.
“An easterly wind direction reduces smoke from stubble burning considerably. It can also help improve visibility to over 2,000 metres during the day,” Srivastava said.