How Cyclone Sitrang helped reduce Diwali pollution in north India

NEW DELHI: A change in the direction of Cyclone Sitrang on Sunday played a big role clearing Diwali pollution over Delhi-NCR, and indeed much of north India, leading to clear skies and relatively clean air quality a day after firecrackers were burst across the region, weather experts said.
Cyclone Sitrang crossed the Bangladesh coast on Monday. Since the system moved in a direction (north-northeasterly) away from the northern plains of India, it pulled winds and moisture towards it.
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This led to moderately strong westerly wind flows and dry conditions, both of which enabled dispersal of pollutants, the experts pointed out.
“A cyclone is a low-pressure system, with winds blowing towards the centre of the cyclone. When Sitrang moved towards Bangladesh, winds from north India moved in that direction. Therefore, it exacerbated the air flow in north India. It also drained out moisture from the region,” said India Meteorological Department chief Mrutyunjay Mohapatra.
Mohapatra added that a deep trough in the mid and upper atmosphere over east Uttar Pradesh also reinforced the westerly wind flows in the region.
As per SAFAR, the Centre's air quality monitoring body, Delhi and NCR reported west-moving winds at "moderate" speeds of 8-16kmph on Tuesday. Dry, westerly winds at moderate to high speeds usually help clear up pollution over north India. Under calmer conditions, however, these winds transport farm-fire pollution from Punjab and Haryana. High moisture in the air too raises pollution levels.
All this underlines the crucial role played by meteorological conditions in either trapping or dispersing pollutants from sources such as Diwali fireworks or stubble-burning.
“Had the cyclone hit the Odisha coast and moved northwestwards towards Chhattisgarh, the situation in north India would have been very different. A cyclone moving towards the region would have led to an incursion of moist, easterly winds, which would have led to cloudy skies. Under such conditions, pollution levels may have shot up,” said Mohapatra.
The cyclone was indeed headed towards the Andhra-Odisha coast before it recurved on Sunday, making instead for the Bangladesh coast. The IMD had accurately forecast the change in direction days in advance.