GUWAHATI, Nov 26 - A climatological study of total lightning flashes, carried out by a team led by Daniel J Cecil from the University of Alabama, Huntsville, USA, has revealed that the highest monthly mean flash rate (18 flashes km2 per month) in the globe occurs over the Brahmaputra Valley of Northeastern India.
The Alabama University team also found that lightning activities varies from month to month and it has observed a sharp peak of lightning activities in April in this area, said Prof PK Bhuyan of the Centre for Atmospheric Studies, Dibrugarh University.
Prof Bhuyan also maintained that several studies have reported about the severe pre-monsoon (spring) thunderstorms, or Nor�westers in the Northeastern part of India, which is associated with intense but short durational rainfall.
Referring further to the findings of the University of Alabama team, Prof Bhuyan said lightning activity shifts North and Westward from this region as summer/monsoon progresses. Lightning activity is negligible in autumn and is nearly absent in winter over this area, he added.
Lightning flash counts are obtained from the lightning sensing instruments, which are placed in the satellites orbiting around the earth. Lightning Image Sensor (LIS) is one of those instruments onboard the Tropical Rainfall Measuring Mission (TRMM) Satellite, which detects the distribution and variability of total lightning that occurs in the tropical regions.
The instrument records the time of occurrence, measures the radiant energy and estimates the location of lightning events within its field-of-view. The satellites have thus facilitated to draw a global picture of lightning flash rate, Prof Bhuyan said.
Recently, a lightning detector has been installed at the Centre for Atmospheric Studies, Dibrugarh University, in collaboration with the Earth Networks, USA as part of a networked programme in NE India and Bhutan for measurement and research on lightning, he informed.
Commenting on the science of lightning, he said thunderstorm is associated with lightning discharges in or between thunderclouds or between thunderclouds and the ground.
Every day millions of lightning activities occur around the globe. Millions of amperes and phenomenal voltages are produced within microseconds in a lightning discharge. Lightning strikes result in hundreds of deaths of people every year, especially in tropical countries, like India, he asserted.
Lightning is a massive electrostatic discharge caused by the circulation of warm moisture-filled air through unbalanced electric field in the atmosphere, accompanied by the loud sound of thunder, he said. Apart from the direct consequences, lightning induces a series of chemical reactions, which influence the atmosphere and may affect human beings through production of tropospheric ozone.
Von Liebig in 1827 had first proposed that lightning plays a major role in the global nitrogen cycle. The chemical reactions of lightning that lead to the production of nitrogen oxide were described by Zel�dovich and Raizerin in 1967 for the first time.
Near or within the hot lightning channel {about 30,000 Kelvin (K)}, oxygen and nitrogen are partly dissociated to provide atomic oxygen and nitrogen, which combines with oxygen to give ozone and nitrogen oxide. Oxides of nitrogen are precursors of ozone in the troposphere. Lightning activities thus contribute to ozone level directly by producing ozone as well as by producing its precursor nitrogen oxides, said Prof Bhuyan.
Tropospheric ozone, though it represents only 10 percent of the total ozone column, is important because it can influence climate, as it is a greenhouse gas.
The effect of lightning in the level of ozone and nitrogen oxides have been observed over Dibrugarh, located in the upper Brahmaputra Valley, in a research work carried out by the Centre for Atmospheric Studies, Dibrugarh University. Spring and summer seasons showed high levels of ozone, which were in part due to frequent lightning activities occurring in these seasons.
During lightning activity, there are sudden peaks in both nitrogen oxides and ozone. Lightning induced about 33 per cent of the rise in the level of ozone near the surface, raising the level of ozone particularly during night time, said Prof Bhuyan.
