Everyone has seen lightning and marveled at its power. But despite their frequency – some 8.6 million occur worldwide each day – why they move in a series of jumps and pauses from the storm cloud to the ground remains a mystery.
There are a few books on this phenomenon, but none have explained how these zigzags (called steps) are formed and how lightning can travel for miles. My new research offers an explanation.
From the sky to the ground, by leaps
The intense electric fields of the thunderclouds excite the electrons until they have enough energy to create what are known as singlet-delta oxygen molecules. These molecules and electrons accumulate to create a short, highly conductive step, which glows brightly for a millionth of a second.
At the bottom of the step, there is a pause as the buildup occurs again, followed by another bright, flickering jump. The process is repeated over and over again.
The increase in extreme weather events makes lightning protection increasingly important. Knowing how they form allows you to develop solutions to better protect buildings, aircraft and people.
Also, while the use of environmentally friendly composite materials in aircraft is improving fuel efficiency, these materials increase the risk of lightning damage, so we need to look for additional protection.
How do lightning strike?
Lightning occurs when storm clouds with an electrical potential of millions of volts connect to the ground. A current of thousands of amperes flows between the ground and the sky, with a temperature of tens of thousands of degrees.
Photographs of lightning reveal a series of details that are not visible to the naked eye. Usually there are four or five weak tracers coming out of the cloud These branch and zigzag in an irregular trajectory towards the earth.
The first of these tracers to hit the ground initiates the lightning strike. Then the others go extinct.
Fifty years ago, high-speed photography revealed even greater complexity. The tracers move downward from the cloud in steps about 50 meters in length. Each step becomes bright for a millionth of a second, but then there’s almost total darkness. After another 50 millionths of a second another step forms at the end of the previous one, but the previous steps remain dark.
Why do these jumps occur? What happens in the periods of darkness between them? How can the rungs be electrically connected to the cloud without there being a visible connection?
The answers to these questions lie in understanding what happens when an energetic electron collides with an oxygen molecule. If the electron has enough energy, it excites the molecule to the singlet-delta state. It is a metastable state, which means it is not perfectly stable, but it does not typically drop to a lower energy state for about 45 minutes.
Oxygen in this singlet-delta state sheds electrons (necessary for electricity to flow) from negative oxygen ions. These ions are almost immediately replaced by electrons (carrying a negative charge) which bind again to the oxygen molecules. When more than 1% of the oxygen in the air is in the metastable state, the air can conduct electricity.
Thus, light steps occur when enough metastable states are created to shed a significant number of electrons. During the dark part of a step, the density of metastable states and of electrons increases. After 50 millionths of a second, the step can conduct electricity, and the electrical potential at its end increases to about that of the cloud, producing a new step.
The excited molecules created in the previous jumps form a column up to the cloud. The entire column is then electrically conductive, without the need for an electric field and with little light emission.
Protection of people and property
Understanding how lightning is formed is important in designing the protection of buildings, aircraft and also people. While it is rare for lightning to strike an individual, buildings are struck many times, especially tall and isolated ones.
When lightning strikes a tree, the sap inside it boils and the resulting steam creates pressure that splits the trunk open. Similarly, when lightning strikes the cornice of a building, the rainwater that has seeped into the concrete boils. The pressure bursts the entire cornice of the building, increasing the risk of fatal collapses.
The lightning rod, invented by Benjamin Franklin in 1752, is basically a thick metal pole attached to the top of a building and connected to the ground. It is designed to attract lightning strikes and ground electrical charge. By directing the flow through the cable, it prevents the building from being damaged.
These lightning rods are needed for tall buildings, but we don’t yet know how many are needed for each construction.