On Tuesday morning, general visibility reduced to 500 meters in Delhi, Agartala and Kolkata. Visibility in Allahabad and Amritsar dropped to 800 meters while in Gorakhpur, Lucknow and Varanasi it remained nil.
Normally, Western Disturbances start bringing precipitation over North India by the second week of December, which leaves enough moisture to get condensed due to nocturnal cooling under clear sky conditions.
Temperature keeps falling in the wake of Western Disturbances, due to penetration of cold and dry northwesterly winds. Consequently, fog starts becoming thicker and if one Western Disturbance (WD) is not followed by another one in quick succession, the cold and dry condition continues unabated. With temperatures well below normal, cold wave conditions engulf entire North India. This results in a vicious cycle, wherein temperatures do not rise because of thick fog. Similarly, as temperatures don’t rise, fog does not dissipate. This cycle is broken when strong winds or precipitation occurs.
This year winter season has been dry so far. This is mainly because WDs have been moving across higher Indian latitudes but they were not active enough to bring good winter rain or cold wave conditions. Temperatures over northwest India have been above normal, which is not favourable for formation of fog.
In the normal course of action, northwest India is the first region to witness fog and then it gradually penetrates from west to east. Due to above normal temperatures in northwest India, this time fog first appeared in East Uttar Pradesh in general and Gorakhpur in particular.
Gorakhpur is one of the most favoured places for winter fog. As it lies in the foothills, warm Katabatic winds flow downslope at the night, which is just the right trigger for fog formation. Secondly, Gorakhpur has a huge pond (sarovar) along with the River Rapti. Warm and light southeasterly winds blowing from these water bodies lead to condensation of surplus moisture. Thirdly, temperature inversion over the area provided the lead for pollutants to remain suspended in the air, giving rise to fog. Gradually, the belt of fog spread to Patna, Gaya and further towards Lucknow and Allahabad.
This layer of fog would dissipate only when there are very strong winds or precipitation. Neither of which is likely before the coming weekend. However, it may be noted that this spell of rain will only temporarily dislodge fog. It will very soon come back with vengeance once the WD moves away and cold dry air sweeps Gangetic plains.