At the end of “Lightning Strikes Same Building Twice, Ignores Famous Saying”, Mr Bhalerao, having accepted that his roof was not the safest place in Pune, announced a new plan. In the next storm, he would stand under the tallest tree in the society garden, “since the tree has never been hit”. His neighbours were leading him inside again. They were right to. To see why, you need to know exactly how a bolt of lightning decides where to land, and the surprising answer is: only at the very last moment.
A lightning strike doesn’t come down in one go. First, a faint channel of charged air, called a stepped leader, creeps down from the cloud in a series of jumps. Each step is roughly fifty metres long, and between steps it pauses for tens of millionths of a second, as if feeling its way. It branches as it goes, sending out tentative forks in different directions. It’s almost invisible, and the whole descent takes a few hundredths of a second. Crucially, at this stage the leader has no idea where it will end up. It is not aiming at anything.
What changes things is the ground getting involved. As the negatively charged leader gets closer, the electric field near the ground beneath it grows enormously, and it’s strongest around anything tall and pointed: trees, poles, the corners of buildings, the top of a person’s head. When the field around one of those points gets strong enough, the air there breaks down and an upward spark, called a streamer, leaps up towards the leader. Several objects may send up streamers at once. The first streamer to meet the leader completes the path, and the main flash, the brilliant return stroke, races along it. Everything else is spared.
The distance at which this connection happens is called the striking distance. It depends on how much charge the leader carries, but it’s typically a few tens of metres, longer for bigger strikes. That’s the key to the whole puzzle. Lightning doesn’t choose the tallest thing in the city. It chooses the tallest, closest thing within a few tens of metres of wherever its wandering tip happened to arrive.
Engineers turned this into a lovely piece of geometry called the rolling sphere method. Imagine a giant invisible ball, its radius equal to the striking distance, often taken as somewhere between twenty and sixty metres depending on how much protection is needed. Roll it over the landscape in every direction, letting it rest on top of buildings, trees and poles. Every point the ball can touch is a point the leader’s tip could reach first, so it could be struck. Every point tucked underneath, where the ball can’t get in, is protected. That’s how lightning conductors are positioned: you put the rods where the sphere touches, so that it touches nothing else.
Now roll the sphere over Mr Bhalerao’s garden. The tallest tree, standing alone in the open, gets touched: it’s a classic strike point. And Mr Bhalerao, standing next to it, is in deep trouble, in three ways.
The first is the strike itself, which may simply hit the tree. Wet wood is a poor conductor compared with a lightning rod’s thick cable, so the current may not stay inside the trunk. The second danger is called a side flash. A person standing close to the trunk offers the current a better path to the ground than the rest of the tree does, and the current jumps sideways through the air into them. The third is the ground. When the tree is struck, the current pours out into the soil in all directions from its roots, and the voltage in the ground falls off with distance from the trunk. A person standing with their feet apart has one foot at a higher voltage than the other, so current runs up one leg and down the other. Ground current is one of the most common ways lightning injures people, and it’s why cattle sheltering under a single tree are sometimes killed in groups.
So what should he do instead? Get inside a proper building with wiring and plumbing, which give the current a path around its occupants, or into a car with a metal roof, where the current flows around the metal shell into the ground. Not because of the rubber tyres, which a bolt that has just crossed kilometres of air will hardly notice, but because of the metal. And if he’s caught in the open, the advice is to avoid being the tallest thing around, keep away from trees and poles, and keep his feet together.
Lightning, in other words, is lazy and short-sighted. It wanders down without a plan and, in its last few tens of metres, grabs whatever is tallest and nearest. The trick to staying safe is simple: never be that thing, and never stand next to it.