Air Quality Index (AQI) of Delhi NCR was somewhat better this December as compared to previous years. Although there were occasional bad patches, but overall air quality remained in moderate to satisfactory category for the longer duration. First 4-5 days of December, the air quality was in satisfactory category. Air pollution over Delhi NCR reduced significantly between December 14 and 19 due to continuous flow of moderate to strong winds from north west direction.
Whenever there is a prolonged gap between two western disturbances, moderate dry and cold winds continue from north west direction leading to dispersal of pollutants. This is what happened in the last week over Delhi NCR. Many localities of Delhi and NCR witnessed air quality index below 100. Most of the time, AQI hovered between 100 and 200.
Since December 20, Delhi has been witnessing gradual increase in pollution levels. Today overall AQI for Delhi NCR is around 485 which comes in severe category. In fact, most of the localities of Delhi NCR are now in severe air pollution category. The moderate winds which are blowing from north west direction has reduced in speed and they are almost calm during evening, night and morning hours. The low temperature, the light and variable winds and high humidity leads to formation of shallow fog and mist. The local pollutants such as dust particles, smoke particles and all the harmful gases gets deposited on the water vapor available in the fog. These pollutants when mixes with fog leads to formation of smog. In absence of moderate winds, the smog layer keeps lingering near the Earth's surface for longer duration and results in deterioration of air quality. The successive western disturbances which are about to approach western Himalayas will not allow the north westerly winds to setting over Northern Plains including Delhi NCR for next few days. Therefore, AQI of Delhi NCR as well as many parts of north India will remain in very poor to severe category for next 4 to 5 days.
The two weather parameters, which can help in reducing pollution are moderate winds from a particular direction for prolonged period or rainfall. There is a possibility of light rainfall around December 27 over northern plains. But this rainfall will neither be widespread nor intense. Chances of washing away the pollutants are bleak. Wind speed is also expected to remain low. Therefore, the air pollution will persist over Delhi NCR for the next 4 to 5 days at least.
