After a short relief, air quality deteriorated again on Thursday due to brief spell of rain on Thursday. The overall Air Quality Index (AQI)  was recorded at 314 (very poor).

According to environment experts, "High humidity and light winds affect dispersion of pollutants and lead to the formation of more potent secondary particles". "Secondary particles are products of complicated atmospheric reactions between primary particles, such as particulate matter, sulphur dioxide and nitrogen dioxide directly emitted by stubble burning and vehicles, in the presence of other factors such as sunlight and moisture," Centre's monitoring authority SAFAR said.      

According to System of Air Quality and Weather Forecasting And Research (SAFAR), a change in wind direction back to north-westerly and plume intrusion is expected only by early November 9.

Gufran Beig, the Director of  SAFAR, said scanty rain in cold weather is always harmful as it leads to high humidity which, in turn, creates secondary particulate matter.

The air quality also took a hit in neighbouring Noida (328), Ghaziabad (325), Greater Noida (318), and Faridabad (312).

However, the weather experts said, Haryana and Punjab received good rains which will reduce the impact of stubble burning in Delhi's pollution. The wind speed will increase again Friday evening onwards, flushing  out pollutants, they said.

Mahesh Pawalat, a senior scientist at Skymet Weather - a private forecaster, said, "We are expecting cold, dry winds to blow from Jammu and Kashmir and Himachal Pradesh due to snowfall. Dry winds will reduce humidity. The air quality will start improving again after 24 hours."  Light rains on Sunday had resulted in an increase in humidity, triggering a massive pollution spike.