June 24, 2016 12:40:57 am
It is a frequent occurrence, and accounts for the largest number of accidental deaths due to natural causes every year — still, lightning is one of the least studied atmospheric phenomena in India. Just one group of scientists, at the Indian Institute of Tropical Management (IITM) in Pune, works full-time on thunderstorms and lightning. Occurrences are not tracked, and there is paucity of data. Safety measures and precautions against lightning are not publicised the same way as those for other natural disasters like earthquakes.
How lightning strikes
Lightning is a very rapid and massive discharge of electricity in the atmosphere, some of which is directed at the earth’s surface. These are generated in moisture-carrying clouds about 10-12 km tall. The base of these clouds is typically 1-2 km from the Earth’s surface, while the top is 12-13 km away. Temperatures at the top are –35 to – 45 °C.
As water vapour moves upward in the cloud, decreasing temperatures causes it to condense. The heat generated in the process pushes the water molecules further up. As they move beyond zero degrees, water droplets change into small ice crystals. As they continue to move up, they gather mass — until they are so heavy that they start to fall.
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This leads to a system where smaller ice crystals move up while bigger crystals come down. The resulting collisions trigger the release of electrons, in a process very similar to the generation of electric sparks. The moving free electrons cause more collisions and more electrons, as a chain reaction ensues.
The process results in a situation in which the top layer of the cloud gets positively charged while the middle layer is negatively charged. The electrical potential difference between the two layers is huge, of the order of 109 or 1010 volts. In little time, a huge current, of the order of 105 to 106 amperes, starts to flow between the layers. It produces heat, leading to the heating of the air column between the two layers of cloud. It is because of this heat that the air column looks red during lightning. The heated air column expands and produces shock waves that result in thunder.
From cloud to Earth
Earth is a good conductor of electricity but is electrically neutral. In comparison to the middle layer of the cloud, however, it becomes positively charged. As a result, a flow of current (about 20-15%) gets directed towards the Earth as well. It is this current flow that results in the damage to life and property.
There is a greater probability of lightning striking tall objects such as trees, towers or buildings. Once about 80-100 m from the surface, lightning tends to change course to hit the taller objects. This is because travelling through air, which is a bad conductor of electricity, electrons try to find a better conductor, and also the shortest route to the relatively positively charged Earth’s surface. The US National Oceanic and Atmospheric Administration says the Empire State Building in Chicago is hit by lightning nearly 100 times a year.
Several thousand thunderstorms occur over India every year. Each can involve several, sometimes more than 100, lightning strikes. Sunil Pawar of IITM says incidents of lightning have been showing an increasing trend over the last 20 years, especially near the foothills of the Himalayas.
People are rarely hit directly by lightning. But such strikes are almost always fatal. The most common way in which people are struck by lightning are by ‘ground currents’. The electrical energy, after hitting a tree or any other object, spreads laterally on the ground for some distance, and people in this area receive electrical shocks. It becomes more dangerous if the ground is wet, or there is conducting material like metal on it.
Prediction and precautions
The Met office routinely issues warnings for thunderstorms. But this is a very generic advisory, for locations that are very large in area. Predicting a thunderstorm over a very precise location is not possible. Nor is the exact time that it is likely to strike. People are advised to move indoors in a storm. Moving under a tree or lying flat on the ground can increase risks. Even indoors, electrical fittings, wires, metal and water must be avoided.
THE WORLD’S MOST ELECTRIC PLACE
The most lightning activity on Earth is seen on the shore of Lake Maracaibo in Venezuela (pictured). At the place where the Catatumbo river falls into Lake Maracaibo, an average 260 storm days occur every year, and October sees 28 lightning flashes every minute — a phenomenon referred to as the Beacon of Maracaibo or the Everlasting Storm. The reason probably lies in the topography of the spot: winds blow across Lake Maracaibo — the largest in South America — which is surrounded by swampy plains and connected to the Gulf of Venezuela/Caribbean Sea by a very narrow strait. The Maracaibo plain is enclosed on three sides by high mountain sides into which air masses crash. The heat and moisture picked from the swampy plains creates electrical charges and, as the air is destabilized at the mountain faces, thunderstorm activity — characterised by almost non-stop lightning activity within clouds — results.
5 DEADLY STRIKES
Occurs most often in open areas; not very common, but the deadliest. Victim is part of main lightning discharge channel. Some current moves along/just over skin surface; some through the body, usually through the cardiovascular and/or nervous systems.
Side Flash (Or Side Splash)
Occurs when lightning strikes a taller object and some current jumps on to the victim, who ends up acting as a “short circuit” for the energy. Generally occurs when the victim is within a foot or two of the struck object. Most victims are those sheltering under a tree in a rainstorm.
When an object is struck, much of the energy travels outward in and along the ground surface. This is ‘ground current’, and anyone close can be a victim. Ground current affects a larger area than other kinds of current and causes the most lightning deaths and injuries. Lightning enters the body at the contact point closest to the strike, travels through the cardiovascular and/or nervous systems, and exits at the point farthest from the strike. Greater the distance between the contact points, greater the potential for death or serious injury. Large farm animals are especially vulnerable.
Lightning can travel long distances in wires or other metal surfaces. Most indoor lightning casualties and some outdoor casualties are due to conduction. Whether inside or outside, anyone in contact with anything connected to metal wires, plumbing, or metal surfaces that extend outside is at risk.
Not too common. “Streamers” develop as the downward-moving leader approaches the ground. Typically, only one of the streamers makes contact with the leader as it approaches the ground and provides the path for the bright return stroke; however, when the main channel discharges, so do all other streamers in the area. A person who is part of one of these streamers, could be killed or injured. Source: NOAA
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