Everyone saw the lightning and marveled at its power. But despite its frequency – around 8.6 million lightning strikes occur worldwide every day – why lightning proceeds in a series of stages from the thundercloud to the ground below has remained a mystery.
There are a few textbooks on lightning, but none have explained how these “zigzags” (called steps) are formed and how lightning can travel for miles. My new research provides an explanation.
The intense electric fields in thunderclouds excite electrons so they have enough energy to create what are called “delta singlet oxygen molecules”. These molecules and electrons accumulate to create a short, highly conductive stage, which fires intensely for a millionth of a second.
At the end of the stage, there is a pause as the buildup repeats, followed by another bright, flashing jump. The process is repeated again and again.
An increase in extreme weather events means that lightning protection is increasingly important. Knowing how a lightning strike is triggered means we can figure out how to better protect buildings, planes and people. Additionally, while the use of environmentally friendly composite materials in aircraft improves fuel efficiency, these materials increase the risk of lightning damage, so we need to consider additional protection.
What leads to love at first sight?
Lightning strikes occur when thunderclouds with an electrical potential of millions of volts are grounded. A current of thousands of amperes circulates between the ground and the sky, with a temperature of several tens of thousands of degrees.
Photographs of lightning reveal a host of details not observed with the naked eye. Usually there are four or five weak “leaders” coming from the cloud. These are branched and zigzag in an irregular path towards the earth.
The first of these rulers to reach the earth initiates the thunderbolt. The other rulers are then extinguished.
Fifty years ago, high-speed photography revealed even more complexity. The leaders descend from the cloud by “steps” of about 50 meters in length. Each footstep becomes bright for a millionth of a second, then there is almost complete darkness. After another 50 millionths of a second, another step forms, at the end of the previous step, but the previous steps remain dark.
Why are there such steps? What happens in the dark times between stages? How can steps be electrically connected to the cloud without a visible connection?
The answers to these questions lie in understanding what happens when an energetic electron hits an oxygen molecule. If the electron has enough energy, it excites the molecule into the delta singlet state. This is a “metastable” state, which means it’s not perfectly stable, but it doesn’t usually fall into a lower energy state for about 45 minutes.
Oxygen in this delta singlet state detaches electrons (needed for the flow of electricity) from negative oxygen ions. These ions are then replaced almost immediately by electrons (which carry a negative charge) attaching themselves 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, flash stages occur when enough metastable states are created to detach a significant number of electrons. During the dark part of a walk, the density of metastable states and electrons increases. 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 an additional step.
The excited molecules created in the previous steps form a column up to the cloud. The entire column is then electrically conductive, with no electric field requirement and little light emission.
Protect people and property
Understanding lightning formation is important for designing the protection of buildings, aircraft and also people. Although it is rare for lightning to strike people, buildings are repeatedly struck, especially large and isolated ones.
When lightning strikes a tree, the sap inside the tree boils and the resulting steam creates pressure, splitting the trunk. Similarly, when lightning strikes the corner of a building, rainwater that has seeped into the concrete boils. The pressure blasts the entire corner of the building, creating a risk of fatal collapse.
A lightning rod invented by Benjamin Franklin in 1752 is basically a thick fence wire attached to the top of a building and connected to the ground. It is designed to attract lightning and ground the electrical charge. By directing the flow through the wire, it prevents the building from being damaged.
These Franklin rods are needed for high-rise buildings and churches today, but the uncertain factor is the number needed on each structure.
In addition, hundreds of structures are unprotected, including shelters in parks. These structures are often made of highly conductive galvanized iron, which itself attracts lightning, and supported by wooden poles.
The new version of Standards Australia for lightning protection recommends that these shelters be earthed.
Provided by The Conversation
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