Since the last 3 to 4 days, Delhi pollution has increased significantly and is now in the severe category over most places. The reason for the increased AQI index can be attributed to the slowing down of winds.
A series of Western disturbances are approaching Western Himalayas due to which wind direction has changed from northerly Northwesterly to easterly. Moreover, wind speed has also decreased significantly. Therefore pollutants are not being dispersed. The ventilation index is also lower now. Due to a drop in temperatures, air become heavier and pollutants do not lift and remain suspended in the lower atmosphere and near the Earth surface.
Two factors usually help in a dispersion or washing away of pollutants. The first one is moderate to strong winds from a particular direction for a prolonged period. Another is widespread rainfall which helps in washing away the pollutants.
We do not expect both of these to increase for at least the next 3 to 4 days. But there are chances of scattered rainfall over Northern Plains including Delhi and NCR between December 26 and 28. These rain activities may help in washing away the pollutants and improving the air quality index.
We think Delhi may breathe easy by December 27.