- John Locke
No one had ever seen lightning and marveled at its power.
But despite their frequency — some 8.6 million lightning strikes occur around the world each day — why they persist as if by a series of steps from the charged cloud to the ground remains a mystery.
There are a few books on lightning, but none of them explain how these “zigzags” (called steps) are formed, or how lightning is able to travel for miles. But my recent research provides an explanation.
The intense electric fields of the charged clouds excite the electrons until they have enough energy to form what are known as “delta-single oxygen molecules”.
These molecules and electrons aggregate to form a short, highly conductive step, which glows brightly for a millionth of a second.
At the end of the step, there is a pause as the build-up occurs again, followed by another glowing flash of light. The process is repeated several times.
The increase in weather extremes means that lightning protection is becoming increasingly important. Knowing how lightning begins to form means we can figure out how to better protect buildings, aircraft and people.
Moreover, while the use of environmentally friendly compounds in aircraft increases fuel efficiency, it also increases the risk of lightning damage. Therefore, we need to seek more protection.
What causes lightning?
Lightning occurs when clouds charged with electric potentials of millions of volts are connected to the Earth.
A stream of thousands of amperes flows between the earth and the sky, with a temperature of tens of thousands of degrees.
X-rays reveal countless details not noticed by the naked eye. There are usually four or five weak “leaders” emerging from the cloud. It is branching and winding in an irregular path towards the ground.
The first to hit the ground is the one who starts the bolt. Then the other leaders are extinguished.
Fifty years ago, high-speed photography revealed even more complexity. Leaders descend from the cloud downward in “steps” of about 50 metres.
Each step is bright for a millionth of a second, but then there is almost complete darkness. After 50 millionths of a second, a new step forms at the end of the previous one, but the other steps remain dark.
Why are there these steps? What happens in the periods of darkness between degrees? And how can the steps be connected electrically to the cloud without a visual connection?
The answers to these questions lie in understanding what happens when an energized electron strikes an oxygen molecule. If the electron has enough energy, it shakes the molecule, which is in a state called delta singlet.
This is an “unstable” state, meaning it’s not completely stable, but it doesn’t usually drop to a lower energy state for about 45 minutes.
Oxygen in the singleton delta state separates these electrons (necessary for the flow of electricity) from the negative oxygen ions. These ions are then almost instantly replaced by electrons (which carry a negative charge), and bounce back to the oxygen molecules.
When more than 1% of the oxygen in the air is in a steady state, the air can conduct electricity. Radius steps occur when sufficient stable states are created to separate a large number of electrons.
During the dark part of the step, the density of steady states and electrons increases. After 50 millionths of a second, the step can conduct electricity—and the voltage at the end of the step increases to nearly that of the cloud, resulting in a new step.
The vibrating particles created in the previous steps form a pillar in the cloud. Then the entire column is electrically conductive, with no electric field required and little light emission.
How do we protect people and property
Understanding the formation of lightning is important for building and aircraft protection projects and also for people. It is rare for people to be struck by lightning, but buildings are frequent targets, especially tall and isolated buildings.
When lightning strikes a tree, the sap inside boils and the resulting vapor builds up, cracking the trunk. Similarly, when lightning strikes a building, rainwater that seeps into the concrete blisters. The pressure can cause part of the building to burst, creating a collapse hazard.
The lightning rod invented by Benjamin Franklin in 1752 is basically a thick wire of fencing attached to the top of a building and connected to the ground. It is designed to attract lightning and deliver electrical charges to Earth. By directing the electrical flow through the wire, it prevents damage to the building.
Currently, Franklin lightning rods are required in tall buildings and churches, but what is not clear is the number required in each building.
In addition, there are hundreds of unprotected buildings, including shelter covers in parks. These structures are often made of highly conductive, lightning-attracting galvanized steel, supported by wooden poles.
In Australia, the new edition of the Standards Required for Lightning Protection recommends that these shelters be grounded.
John Locke is a professor and researcher in physics at the University of South Australia
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