There is a strong connection between Western Disturbances and dense fog over Northern Plains including Delhi. When a strong Western Disturbance approaches Western Himalayas and dumps heavy snowfall and subsequently induces a Cyclonic Circulation over Northern Plains, it leads to winter rain. Rain activity increases the humidity levels leading to the formation of fog. Low temperature, high humidity and light winds are ideal weather parameters for fog formation.
In the absence of winter rain, humidity factor is still lacking. However, due to low temperatures and light winds air quality index of Delhi and NCR has become a very poor once again. Shallow fog is when visibility drops below 1000-meter, moderate fog is when visibility remains between 200 and 500 meter and dense fog is when visibility drops below 200 meters.
The visibility dropped to 800 metres over parts of Delhi on Sunday but Delhi has not witnessed a single dense fog hour until now. Last year, there were 90 fog hours over Delhi during December but only one dense fog hour. Similar conditions are expected this year as well.
We expect light rain to occur over northern plains which may increase the humidity percentage over Delhi. Therefore, chances are bright for dense fog to appear by month end over Northern Plains including Delhi and NCR.
