GUWAHATI, April 10 - Northeast India, together with Bangladesh, is one of the most thunderstorm-prone regions in the world. This was found by Tetsuya Fujita of the University of Chicago in 1973. Fujita along with Allen Pearson had developed the Fujita Pearson Scale for measuring damage caused by tornados.
Disclosing this, Dr Rahul Mahanta, Associate Professor, Interdisciplinary Climate Research Centre (ICRC), Cotton University, who is doing a study on the thunderstorm and lightning events of the northeastern region, said the districts with highest frequency of severe thunderstorm days in the region are located in the Brahmaputra valley. Central Assam is the most affected part of the valley in this regard. In comparison, the Barak valley has a lower frequency of severe thunderstorm days.
Dr Mahanta is doing the study together with Dr Yusuke Yamane, Associate Professor, Tokoha University, Japan, on different aspects of severe thunderstorms over Northeast India. They have been making the study on a 55-year period between 1962 and 2016.
He said a total 454 cloud and ground (CG) lightning strikes were observed over Assam for the entire period of their study, with a mean of 12.6 CG lightning strikes per year. The total climatology of lightning activity showed that the western region of Assam experiences higher lightning activity. The minimum lightning activity is found to be occurring in northeastern and eastern parts of the region.
Summer monsoon season with 60 per cent events is the most favoured time of the year for occurrence of lightning strikes in Assam, followed by pre-monsoon season with 32 per cent of the events.
Of the total 595 hail occurrences, pre-monsoon season accounts for over 80 per cent of these hailstorm events, followed by the winter season (December, January and February) with 13.7 per cent of these events. West of Assam accounts for majority of the events with a minimum of them recorded in the State�s northeastern part.
During the 55-year study period, it was found that 22 people died on an average per year from severe thunderstorm hazards in Northeast India. More than 60 per cent of these death cases were due to lightning, said Dr Mahanta.
In general, severe thunderstorm impacts like loss of life and injury, loss of livelihood and damage to infrastructure are significantly more severe on impoverished and vulnerable rural population in the western parts of Assam.
Of all the severe thunderstorm events in the region during the 55 years of study period, about 30 per cent of the events resulted from squalls (nor�westers), with hail and lightning accounting for 18 per cent and 10 per cent of all recorded events.
While severe thunderstorms can and do develop at any time of the year, over half of the severe thunderstorm events occurred in the region during March, April and May, peaking during the latter months. A secondary peak in severe thunderstorm events occur in September, and is likely due to the impact of tropical cyclones or their remnants flowing from the warm waters of the Bay of Bengal.
More than half (53 per cent) of all severe thunderstorm events occurred between the afternoon hours of 3 pm and 8 pm of the Indian Standard Time (IST), with the activity peaking between 5 pm and 7 pm (IST). It has been found that diurnal variation of severe thunderstorms depends on destabilisation due to solar heating.
The data compiled by the ICRC on the occurrence of severe thunderstorm incidents show that they are first seen on an isolated day in the month of February under the influence of a western disturbance, and it becomes a familiar feature during the hot afternoons of April to May to early morning hours of the next days, said Dr Mahanta.