From UPSC perspective, the following things are important :
Prelims level : Lightening
Mains level : Lightening mechanism
- Twenty-nine people have been killed by lightning over the past 36 hours in Bihar.
Deaths by Lightening
- Lightning is the biggest contributor to accidental deaths due to natural causes.
- A few years ago, over 300 people were reported killed by lightning in just three days — a number that surprised officials and scientists.
- And yet, lightning remains among the least studied atmospheric phenomena in the country.
- Just one group of scientists, at the Indian Institute of Tropical Management (IITM) in Pune, works full-time on thunderstorms and lightning.
What is lightning?
- Lightning is a very rapid — and massive — discharge of electricity in the atmosphere, some of which is directed towards the Earth’s surface.
- These discharges are generated in giant moisture-bearing clouds that are 10-12 km tall.
- The base of these clouds typically lies within 1-2 km of the Earth’s surface, while their top is 12-13 km away.
- Temperatures towards the top of these clouds are in the range of minus 35 to minus 45 degrees Celsius.
How does it strike?
- As water vapour moves upward in the cloud, the falling temperature causes it to condense. Heat is generated in the process, which pushes the molecules of water further up.
- As they move to temperatures below zero degrees celsius, the water droplets change into small ice crystals.They continue to move up, gathering mass — until they are so heavy that they start to fall to Earth.
- This leads to a system in which, simultaneously, smaller ice crystals are moving up and bigger crystals are coming down.
- Collisions follow, and trigger the release of electrons — a process that is very similar to the generation of sparks of electricity.
- As the moving free electrons cause more collisions and more electrons, a chain reaction ensues.
- This 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 a billion to 10 billion volts.
- In very little time, a massive current, of the order of 100,000 to a million amperes, starts to flow between the layers.
- An enormous amount of heat is produced, and this leads to the heating of the air column between the two layers of the cloud.
- This heat gives the air column a reddish appearance during lightning. As the heated air column expands, it produces shock waves that result in thunder.
How does this current reach the Earth from the cloud?
- While the Earth is a good conductor of electricity, it is electrically neutral.
- However, in comparison to the middle layer of the cloud, it becomes positively charged.
- As a result, about 15%-20% of the current gets directed towards the Earth as well.
- It is this flow of current that results in damage to life and property on Earth.
- There is a greater probability of lightning striking tall objects such as trees, towers or buildings.
- This happens because air is a poor conductor of electricity, and electrons that are travelling through air seek both a better conductor and the shortest route to the relatively positively charged Earth’s surface.
When does lightening hit people?
- Lightning rarely hits people directly — but such strikes are almost always fatal.
- People are most commonly struck by what are called “ground currents”.
- The electrical energy, after hitting a large object (such as a tree) on Earth, 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 (which it frequently is because of the accompanying rain), or if there is metal or other conducting material on it.
- Water is a conductor, and many people are struck by lightning while standing in flooded paddy fields.
How uncertain is its prediction?
- Predicting a thunderstorm over a pinpointed location is not possible.
- Nor is it possible to predict the exact time of a likely lightning strike.
Precautions to be taken
- For reasons given above, taking shelter under a tree is dangerous.
- Lying flat on the ground too, can increase risks.
- People should move indoors in a storm; however, even indoors, they should avoid touching electrical fittings, wires, metal, and water.