Hours after pollution levels in Mumbai suddenly spiked to make it more polluted than Delhi, wind conditions shifted, leading to clearer skies. The highest pollutant recordings now come from Lucknow, Pune and then Delhi, according to limited data from the Central Pollution Control Board.

Stagnant winds over four days and moisture pulled in from a western disturbance have contributed to soaring pollution levels in western India, including Mumbai and Ahmedabad.

Cool winds from the Arabian Sea and the east converge in central India around February and March, explained Gufran Beig, program director of the Safar, an air quality forecasting and research organisation associated with the Indian Institute of Tropical Meteorology that monitors air pollution in four cities. Since there were stagnant winds over western India at the same time, pollutants in cities such as Ahmedabad and Mumbai also began to build up, he explained.

“In winter, air quality is normally moderate to poor,” Beig added. “But the day temperature is now increasing, which was not case two days back, so it is likely by tomorrow things will become better.”

The situation in Mumbai rapidly improved over Thursday. After a peak PM2.5 reading of 455 at one station around 10 am, by evening the reading had come down to 236. PM2.5 refers to minute suspended particulate matter with a diameter of less than 2.5 micrometers. This is strongly linked to respiratory diseases.

Lucknow, like other parts of northern India, continues to be beset by fog, trapping pollutants. But its bad air quality is not an anomaly. A study released in January showed it had only seven days of clean air in 2017.

Unlike Delhi or Lucknow’s landlocked geography, the sea breeze in Mumbai helps to disperse pollutants, making it unusual for the western city to be more polluted than cities in the north. Among the common sources of pollution across cities are construction activities, vehicle emissions, open burning and pollutants from industrial activities.

On January 25, the Supreme Court had asked the Centre to examine the issue of air pollution on a national level, citing reports that cities such as Raipur and Patna were far more polluted than the national capital.

Any measures to improve this might not prove fruitful, said Sharad Gokhale, a researcher at the Indian Institute of Technology at Guwahati studying the presence black carbon in Guwahati’s atmosphere.

“Activities are growing at such a fast rate that whatever measures government or local bodies make prove counter-productive,” Gokhale said. “For traffic, emissions standards are made stringent from time to time, and fuel efficiency and engine technology are talked about. But so many vehicles are also added every day.”

While governments cannot manage fog, Gokhale added, they could address the sources of pollution.