The weather gods came to Delhi’s rescue a day after Diwali, helping the city get a breather from the incessant cracker bursting on Saturday. While Delhi’s air quality index remained in the ‘severe’ zone on Sunday, high wind speed and rain in various parts of the city helped improve the air. This improvement is expected to continue till Monday when the AQI levels are likely to come back to the ‘very poor’ zone, the weather forecast said.

Senior scientists at India Meteorological Department (IMD) said that a western disturbance, which started impacting the Western Himalayas on Saturday, brought widespread and moderate rains to Delhi NCR. Delhi recorded 0.4mm rainfall till 5.30 pm (at Safdarjung observatory) on Sunday and average wind speed over the city touched 40kmph by evening.

Kuldeep Srivastava, head of IMD’s regional weather forecasting centre, said that the wind direction blowing over Delhi changed from north-westerly to easterly, which also reduced the transport of stubble smoke from Punjab and Haryana.

“At the Safdarjung observatory, the maximum temperature was recorded 29.1 degree Celsius, while the minimum temperature was 11.4 degree Celsius, which is two degrees below the season’s normal. Wind speed will remain high in Delhi and NCR throughout Sunday night, which will help in the dispersion of pollutants,” Srivastava said.

IMD recordings show that the Palam observatory recorded 1.8mm rainfall, Ridge recorded 1.2mm rainfall and at Jafarpur and Najafgarh, 1mm rainfall was recorded till 6.30pm on Sunday.

Unlike previous years when air pollution levels remain very high turning to “severe” category the day after Diwali, this time air pollution levels gradually reduced on the day after Diwali due to favourable meteorological conditions.

For instance, in 2019, Diwali was on October 27. The air quality index (AQI) on October 27 and 28 were 337 and 368 respectively both in “very poor” category. But on October 29 and 30 air quality had deteriorated to “severe” category with AQI of 400 and 419.

“Delhi escaped prolonged exposure to severe air pollution this time because of the western disturbance. It has brought rain and thundershowers to many parts of northwest India. The wind speed started picking up since Sunday morning recording over 10 kmph which led to gradual dispersal. Winds were stronger in the evening when it started raining. We expect that air pollution levels will reduce significantly tonight,” said Vijay Soni, scientist at IMD’s air quality division.

The ventilation index on Sunday was approx. 10000 m2/s. The ventilation index is a function of the mixing height and the wind speed and defines the ability of the atmosphere to disperse contaminants. A ventilation index below 2,350 sq metres/second is considered poor.

Mixing height is the height at which pollutants mix in the air.

Winter chill is likely to set in over northwest India in a couple of days due to the impact of the WD. “The minimum temperature is likely to fall in the next couple of days by about 2 to 4 degree C due to cold winds from the Himalayan region,” said M Mohapatra, director general, IMD.

The western disturbance was lying over Afghanistan and neighbourhood on Sunday. Under the influence of the Western Disturbance, an induced cyclonic circulation and its interaction with lower level easterlies, widespread rainfall accompanied with thunderstorm, hailstorm and lightning was recorded over the Western Himalayan region.

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