Bengaluru: Over 100 people were killed in Bihar and Uttar Pradesh in just a day as severe thunderstorms resulted in lightning strikes Thursday. The victims include farmers in open fields and children playing outdoors.
At least 83 people were killed in Bihar and 24 in UP as a fusillade of lightning struck across 31 districts of the two states. The states announced ex-gratia amount of Rs 4 lakh for the victims and asked the citizens to heed the thunderstorms warnings.
Lightning kills over 2,000 people in India each year and is the leading cause of accidental deaths by natural causes. Bihar is especially prone to death by lightning strikes this time of the year during the heavy monsoon season. However, this year’s intensity of lightning strikes has been more severe than normal, say experts.
The mechanism behind lightning
Lightning is an electrostatic discharge that releases tremendous amount of energy, with the rapid movement of electrons converting air to superheated plasma. Thunder that follows is caused by the shockwave experienced by the atmospheric gases near the lightning as they undergo compression due to pressure.
There are three categories of lightning — intracloud (IC), which is flashes within a cloud, cloud to cloud (CC), where lightning jumps from one cloud to another, and cloud to ground (CG), where lightning strikes from the ground. This is the most well understood kind of strike and the riskiest to life.
Scientists do not understand the mechanism of lightning perfectly, but it is known that electrification of clouds occur when warm air mixes with cool air.
A fast, upward movement of air during a thunderstorm, called an updraft, carries with it warm humid air, that turns into super-cooled water droplets and ice crystals as it climbs through the freezing clouds, which are on average 10 km high with temperatures of -40°C at the cloud tops. Larger ice crystals called graupel are heavier and already present in the clouds. As collisions between the rising supercooled water droplets or ice occur with the suspended or falling graupel, the former become positively charged and the latter negatively charged.
The updraft carries the positively charged particles upwards and disperses it horizontally, because of which the top of a storm cloud becomes wide and positively charged. The lower parts of the cloud become negatively charged.
As a thundercloud moves over the electrically neutral ground, an equal but opposite positive electric charge is induced on the ground below because the middle and bottom layers of the cloud is negative by comparison. This is known as an image charge. The induced positive surface charge at a point increases slowly as a cloud approaches. It is strongest with the cloud is directly overhead, about 2 km from the ground, and wanes as the cloud passes.
The negatively charged lower cloud and the positively charged ground create an electric field in the air connecting them, leading to a discharge of electricity.
Lightning strikes and safety
There are multiple ways through which lightning strikes humans, buildings or trees. Lightning prefers to strike tall structures like buildings and trees because air is a bad conductor of electricity as compared to such objects.
In the deadliest form of strike, a person is directly struck by lightning. The electric current can heat and burn the skin, as well as travel to the ground through the nervous system. Direct strikes on humans or animals are rare but often fatal.
Instead, lightning strikes trees with a high frequency. But it’s a myth that lightning cannot strike the same location twice or that there are trees that are immune to it.
A side flash occurs when lightning strikes a nearby tall object, such as a tree, and then jumps from the tree to a human. This happens when a person is within 2 feet or so of the tree, typically because they are taking shelter underneath it against the rain or storm.
Ground current is when lightning strikes an object like a tree and the electric current travels along the ground, especially if there are conductive materials in the floor or on the ground, such as water from rain or flooding. This type of conduction covers the maximum area and is the biggest cause of deaths worldwide by lightning, killing even animals in large numbers.
“In India more than 70% people get killed by lightning when they take shelter below tree during rain,” said S.D. Pawar, senior scientist at the Indian Institute of Tropical Meteorology (IITM), Pune who specialises in atmospheric electricity and runs India’s first lightning detection network.
There are no institutes or research groups dedicated to full time research on lightning, except this one group at IITM.
Lightning also travels large distances through other conducting media like electrical wires, metal surfaces, showers and taps and landline phones. This is why, even though indoor locations are safer, it is important to not touch anything conductive during a thunderstorm.
The same principle of conduction is exploited by lightning rods, the most common way to mitigate lightning strikes in buildings. Lightning rods are metal conductors that simply guide the lightning away from the building, within which it could potentially cause a fire or electrocution.
Lightning can be predicted in a region with a reasonable degree of accuracy, even if the exact point they would strike cannot. Formation of lighting producing clouds, which are called thunderstorms, can be predicted well, explained Pawar.
“It is observed that lightning is always associated with severe convections which can produce heavy rains and sometime floods,” he said.
In the latest incident, the Indian Meteorological Department did issue a severe flood warning along with thunderstorm, lightning, and squall (sudden gusts of stormy winds) for Thursday and Friday.
The network gathers data from 83 sensors placed across the country. “Lightning network consists of lightning sensors and central processing units,” said Pawar.
“Lightning sensors receives the electromagnetic signal generated by lightning discharge and send it to central processing unit which process the information coming from different sensors and estimate the position, intensity and some more parameters of lightning discharges and show the position of lightning strikes on the map. This network can be useful for now casting of thunderstorms,” he said.
The network also supplements an app for warnings called Damini, which has been developed by IITM and is available on the Google Play store.
“Thunder can be heard even when a thundercloud is about 15 km away, and this gives 15 to 20 minutes to move to a safe place,” said Pawar.