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Lightning
Have you ever looked to the sky during a storm and wondered ‘what is lightning?’ Well, lightning is an atmospheric discharge of electricity that is frequently accompanied by thunder when it occurs during a thunderstorm. Lightning can occur during volcanic eruptions and dust storms. The front end of a bolt of lightning can travel 60,000 m/s and reach 30,000°C. That is hot enough to turn silica sand into glass.
Lightning can have a positive or negative charge. It is overwhelmingly negative. An average bolt of negative lightning carries a current 30,000 amperes(amps). Large bolts of lightning can carry up to120,000 amps. Positively charged lightning has about 3,000,000 amps. Lightning rapidly heats the air in its immediate vicinity to about 20,000°C which is about three times the temperature of the Sun’s surface. This compresses the surrounding clear air and creates a supersonic shock wave that decays and becomes an acoustic wave that we call thunder.
The formation of lightning is not completely understood, but theories abound. The most widely accepted one is the electrostatic induction hypothesis, which states that charges are driven apart by as-yet uncertain processes. Charge separation appears to require strong updrafts that carry water droplets upward, supercooling them. These collide with ice crystals to form a soft ice-water mixture. The collisions result in a slight positive charge being transferred to the ice crystals, and a slight negative charge to the ice-water mix. Updrafts drive the lighter ice crystals upwards, causing the cloud top to accumulate an increasing positive charge. Gravity causes the heavier, negatively charged mixture to fall toward the middle and lower portions of the cloud. Charge separation and accumulation continue until there is a lightning discharge.
High speed video has shown that most lightning strikes are made up of multiple strokes. A typical strike is made of 3 or more strokes. ‘re-strike’ is separated by 40 to 50 milliseconds. The stroke usually re-uses the discharge channel taken by the previous stroke. The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes. The sound of thunder is made longer by successive re-strikes.
What is lightning has been a question asked for thousands of years. Scientists do not have a complete understanding of the phenomena, but some hope to harness its energy someday.
We have written many articles about lightning for Universe Today. Here’s an article about lightning storms seen on Venus, and here’s an article about the lightning storms triggered by the Chaiten Volcano eruption.
If you’d like more info about lightning, check out NASA Science Website. And here’s a link to the NASA Lightning and Atmospheric Electricity Research Homepage.
We’ve also recorded an episode of Astronomy Cast about Electromagnetism. Listen here, Episode 103: Electromagnetism.
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