THUNDER storm associated with lightning is a natural phenomenon that we have been familiar with from our very childhood. In fact, it is a display of the release of energy created in the atmosphere. The dazzling flashes of lightning in the sky and the roaring thunders have always scared us. Death caused by lightning strikes was rare in the past but has been on the increase in recent years. The situation is alarming this year as almost every day, the farmers or villagers were killed here and there across the country. The death toll reached 150 in two months of summer.
Storm cloud and lightning
IN EARLY summer, wind direction changes from south to north over Bangladesh. The air full of water vapours from the Bay of Bengal flows over the land to cold region of the Himalayas. The moistures in the air condense and turn into thin particles of water and ice at high altitude of subzero temperature. The temperature differential between upper and lower part of the cloud creates a convective force. The air whirls up and down violently. The tiny particles of ice acquire static charge due to mutual friction. The more the internal convective force, the higher is the generation of static electricity. The particles with positive charge accumulate at the upper region of cloud and those with negative charge at the lower region.
Lightning occurs when the positive and negative charges of the clouds meet together. It is like the flash of short-circuit. Most lightning takes place from cloud to cloud and is noticeable as flickers behind the background. Lightning also takes place from cloud to ground and is visible as a flash of piercing line in the horizon.
There is a theory of how it takes place. The huge negative charges in the cloud influence the ground below. The positive charges on the ground accumulate underneath due to attraction of negative charges of the cloud above. As concentration increases, some charge starts to move up by the wind with dust and moisture. Above in the cloud, a leader of negative charges starts its travel downwards finding its way through the permeable strip of air in a zigzag fashion to prepare a route to the ground. Once the route is established, bulk charges come down in a fraction of a second causing a lightning strike.
Cause of thunder
THUNDER is caused by lightning. The lightning generates extreme heat and the temperature is around 20,000 degrees Celsius. The surrounding air being superheated rapidly expands with propagation of massive shock waves that create the loud sound called thunder. As the speed of sound is much less than that of light, a thunder is heard a few seconds after the lightning is struck. That is why if one hears a thunder, one survives the bolt. The distance of the place of occurrence of lightning can be estimated from the time lag between seeing the lightning and hearing the thunder. For example, if the time difference is 15 seconds, then the lightning was more than five kilometres from the viewer.
Effect of global warming
THE global warming due to greenhouse gas emission aggravated the intensity and the frequency of thunder storm. With the increase in global temperature, the atmospheric air can hold more water vapour than before. The increased amount of water vapour is converted into water and ice particles. Huge amount of latent heat is released that causes temperature imbalance and violent turbulence in air. Tremendous amount of energy of static electricity is generated and released through lightning and bolts. A thunder storm, which would otherwise be normal, becomes a violent one due to global warming.
Lightning in urban area
THE tall buildings in urban area normally have lightning arresters at the roof. These are rods with pointed tips facing the sky placed at the top of the building. They are grounded by copper wire. The positive charges on ground surface travel upward and concentrate at the tips and emit to the atmosphere. According to a theory, the lightning arresters act to divert the strike. Moreover, the power distribution system in urban area is equipped with adequate lightning arresters to absorb the surge due to lightning.
Lightning in rural area
THE open fields of agriculture in countryside are susceptible to strikes of thunder bolts. Because of the attraction of negative charge of storm cloud above, the vast paddy fields, trees and housetops become positively charged. Even the tips of the paddy leaves will carry overwhelming charges. The area becomes favourable for lightning strikes. Vulnerable is a farmer working in the watery field, as he may get electrified if lightning strikes nearby. A direct hit can burn the body to ashes.
How to cope with thunderbolt
THUNDERBOLTS are natural calamity and cannot be resisted. To take precautions is the best way of remaining safe. Seeing the dark cloud in the horizon and hearing the thunder, one must return immediately to house which is a safer place. Better to remain inside for an hour till the storm ceases. To remain outdoors during storm and taking shelter under a tree needs to be avoided. Agricultural workers, if caught by storm in the field, should immediately move to a dry place and sit with head down within knees. A person should not appear as a tall object in open area during storm. The children should not be allowed to go out picking fallen fruits in groves or play in the rain. Fishermen out in the river or haor should return to shore to safe shelter because water is a good conductor of electricity. We are to respect the nature and not abuse or underestimate —it has a tool to electrocute us!
MILLIONS of megawatts of electricity are wasted daily in the thunderstorms taking place in different parts of the world. There is no technology yet to capture and use the electricity of lightning. There is a wishful hope that in future, the technological development will make it possible to harness some electricity of lightning for the benefit of human civilisation. The human race looks forward to seeing an era when the atmospheric electricity will be recovered like the renewable energy.
KM Mahbubur Rahman is a senior mechanical engineer and energy consultant.
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