Excited molecules explain the mysterious zigzag patterns of lightning
Lightning strikes are thought to occur almost 50 times a second around the world, but there is still much for us to learn about these incredibly energetic flashes of light. One particularly longstanding mystery is why lightning takes on a zigzag shape as it flashes across the sky, and scientists in Australia have now come up with an explanation, pinning it on collisions between electrons and molecules that create successions of so-called "steps."
In the past five or so years, we’ve seen researchers gain some fascinating insights into the science of lightning strikes. This includes the discovery that gamma rays generated through these events can create antimatter in the air, evidence that their electromagnetic field could have a protective effect on living cells and stunning high-speed camera footage of lightning branches breaking from the clouds.
Though our knowledge of these weather events continues to improve, the physics of lightning strikes and the sequence of events that underpin them remains unclear. The question of why lightning strikes emanate from clouds and follow a zigzagging path down toward Earth has puzzled scientists for half a century, but plasma physicist Dr John Lowke from the University of South Australia has now put forward what’s described as a definitive explanation.
“There are a few textbooks on lightning, but none have explained how the zigzags (called steps) form, why the electrically conducting column connecting the steps with the cloud remains dark, and how lightning can travel over kilometers,” Lowke says.
As lightning emerges from a thundercloud, it does so in steps around 50 meters (164 ft) in length. These flash brightly for a tiny fraction of a second before going dark for another fraction of a second, before another step forms and flashes brightly for the same, minuscule moment in time. These steps repeat, one after the other until the lightning reaches the Earth, and while largely invisible to the naked eye, high-speed photography has enabled scientists to document this dramatic daisy chain of electrical discharge.
But why do these steps form in the way they do, and how can there be a continuous electrical connection when the lightning strikes undergo fleeting moments of darkness along the way? In newly published research, Lowke and his colleague Endre Szili pin the effect on what are called singlet-delta metastable oxygen molecules. These high-energy molecules are created when electrons collide with oxygen molecules, causing them to detach and redistribute the electric field.
The result is the creation of a conductive column that maintains a connection to the cloud as the steps forge a path toward the ground, even when the strike goes dark. This is facilitated by the excitement of oxygen molecules that force them into a “metastable” state, which, when present in high enough numbers, can enable the air to conduct electricity.
“So the lightning steps occur as enough of the metastable states are created to detach a significant number of electrons,” Lowke writes in an accompanying piece for The Conversation. “During the dark part of a step, the density of metastable states and electrons is increasing. After 50 millionths of a second, the step can conduct electricity – and the electrical potential at the tip of the step increases to approximately that of the cloud, and produces a further step.”
According to the scientists, better understanding of lightning strikes can lead to better protective measures for buildings, aircraft and people. Lightning rods invented in 1752 by the polymath Benjamin Franklin remain our primary measure in this regard, and work by attracting lightning and earthing its electrical charge. Fixed to the top of a building, this can guard the structure from damage, but better knowledge of how lightning strikes take shape could lead to new tools to protect the built environment.
“We need to understand how lightning is initiated so we can work out how to better protect buildings, aeroplanes, skyscrapers, valuable churches, and people,” Dr Lowke says.
The research was published in the Journal of Physics D: Applied Physics.