Scientists blow cover off dense fog in north India

Dense fog cover and below normal temperatures observed in north India are a result of a new high- impact weather phenomenon that affects a large belt in the northern part of the Indian subcontinent, according to weather scientists.

The new phenomenon was observed for the first time in December 1997, and since then it has recurred in December 1998, January 2003, December 2008, January 2009 and January 2010, besides January this year. Analysis of daily images from satellites between 1997 and 2011 has shown presence of identical large- scale dense fog cover over northern parts of India, parts of Pakistan and Bangladesh during these months. The fog cover aligns itself in the shape of the Indo- Gangetic plains in satellite imagery.

One of the widely felt impacts of this phenomenon is that airports across the region are simultaneously hit by fog.

Conventional weather events such as Western Disturbance can’t fully explain the dense fog and the below normal temperatures seen in this region in winter months, according to Dr Rajendra Kumar Jenamani of the Indian Meteorological Department.

The fog covered region has typical geographic features. On the northern side exists the Himalayan range, which not only acts as a source of high cooling but also acts as a wind barrier. On the southern side are plateaus and hills running almost 2,500 km in east- west direction.

On the eastern and western sides, there are narrow openings into the sea.

“ This geography makes it a large tub like structure protected from both sides — which favours calm winds. The narrow openings on the east and the west allow a very high interaction with the sea, bringing moisture which helps fog formation and its persistence,” Jenamani said.

While the geography has remained unchanged, there are other factors which have exaggerated effects of this typical geography. “ Since late 1990s, the entire belt has been experiencing significant rise in urban pollution,” the scientist explained. In addition, the dense canal network of major rivers in the Indo- Gangetic plains provides high moisture.

All this contributes to fog formation. The dense fog clouds are not of uniform depth through the region and sometimes get re- formed or lifted to higher altitude at some points due to strong winds.