Delhi’s air quality on Friday morning remained in the ‘poor’ category but central agencies say it is likely to improve marginally due to favourable wind speed, while the city recorded the lowest temperature in the month of November in 14 years.
The minimum temperature in the city, the India Meteorological Department said, was recorded at 7.5 degrees Celsius, making it the lowest temperature of the month since November 29, 2006, when it recorded a minimum temperature of 7.3 degrees Celsius.
The national capital is also on the brink to experience the first cold wave of this season.
On the other hand, Delhi's air quality index was recorded at 292 at 10 am. The 24-hour average AQI was 283 on Thursday, 211 on Wednesday and 171 on Tuesday.
An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".
The Air Quality Early Warning System of the central government for Delhi said that the air quality should improve and remain in the poor category on Friday and jump to the poor to moderate category on Saturday.
While the maximum wind speed is 15 kmph, the wind direction is northwesterly, it said.
According to the Ministry of Earth Sciences' air quality monitor, SAFAR, stubble burning accounted for 20 percent of Delhi's PM2.5 pollution on Thursday. It was 8 percent on Wednesday and 3 percent on Tuesday.
Pollutants are trapped close to the ground due to calm winds and low temperatures, while they are dispersed due to favourable wind speed.
Delhi's ventilation index -- a product of mixing depth and average wind speed was around 12,000 m2/s on Friday and is likely to be 13,000 m2/s on Saturday.
Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.
A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
The IMD further said that the minimum temperature will drop to 7 degrees Celsius in Delhi by Monday as a result of cold winds that have started to blow from hilly regions, which have received a fresh bout of snowfall.
The IMD declares a cold wave when the minimum temperature is t 10 degrees Celsius or below and is 4.5 notches lower than normal for two consecutive days.
“The criteria has been met on Friday. We will declare a cold wave in Delhi if the situation persists on Saturday,” Kuldeep Srivastava, the head of the IMD's regional forecasting centre, said.
The average minimum temperature from November 22 to November 26 is 11.3 degrees Celsius, according to the India Meteorological Department.