What Causes Lightning And Thunder?

Lightning strikes happen as negative ions from the clouds and positive ions from the ground meet in the air. Photo: nssl.noaa.gov

What causes lightning: When a positive and negative charge grows large enough in the sky you get lightning. This giant spark of electricity surges through clouds and sometimes touches down on Earth’s surface.

What causes thunder: Thunder is the sound lightning makes, this is because lightning suddenly increases the pressure and temperature around it, expanding the air and making a thunderous boom.

Most of us are familiar with thunderstorms and the lightning and thunder that accompany them. However, most of us aren’t all that familiar with how thunder and lightning are produced in a thunderstorm. Put simply, lightning is a result of electrons moving between the clouds and the ground, and thunder comes as a result of lightning.

Atmospheric Static

Scientists are still trying to work out exactly how lightning forms, but we know it has something to do with the interactions between positive and negatively charged ions within clouds.

If you’ve ever walked across a carpet and then touched a doorknob only to get shocked, you’ve experienced a discharge of static electricity. Your movement across the floor generated a static (non-moving) electric charge, which looks for this first possible opportunity to escape and discharge. The same thing happens with lightning inside clouds, just on a much larger scale.

The crystals of ice and water that constitute clouds are water molecules, and during a storm the air moves these particles around, making them collide. According to some current theories on lightning formation, the negatively charged electrons from the upward-moving molecules of water are transferred to the heavier molecules that are moving downward. This situation means that the top of the storm cloud is positively charged and the bottom of the storm cloud is more negatively charged.

A Flash of Lightning

Since like charges repel each other (think about the same sides of magnets repelling), the negatively charged ions on the Earth’s surface are pushed away, leaving the ground below the storm cloud with an overall positive charge. This forms an electric field in-between the ground and the base of the cloud. This electric field allows for the negative ions to discharge down to the ground, while the positive charges move upward.

Lightning bolts form as the negatively charged ions from the bottom of clouds and the positively charged ions from the ground meet in mid air. Photo: knuttz.net

The positive ions move towards the tallest objects around like trees and telephone poles, trying to get as close as possible to the sky. As for the negative ions, they move downward rapidly in a precursor to lightning called a “stepped leader”. This process happens very quickly, in only milliseconds, so it is too fast for the human eye to see. The two groups of ions meet in the middle, creating a brilliant flash of lightning. While this is how cloud-to-ground lightning strikes occur, lightning bolts can also move from cloud-to-cloud.

Cloud-to-cloud lightning is much more common than cloud-to-ground lightning. Photo: nssl.noaa.gov

Cloud-to-cloud lightning happens when positively and negatively charged ions interact within areas of the cloud, ignoring the ground. Usually, it happens within opposite halves of the same cloud, but sometimes it can shoot from cloud to cloud. Cloud-to-cloud lightning is actually the most common type of lightning, with cloud-to-ground lighting happening around ten times less frequently than cloud-to-cloud lightning.

Lightning Phenomena

Not only does lightning create an impressive flash, it also creates a lot of heat. A bolt of lightning heats up the air around it at 25,000 degrees Celsius, or about 45,000 degrees Fahrenheit. This is about five times hotter than the surface of the sun, though of course it only lasts for a fraction of a second.

Most cases of lightning strikes are “negative lightning”, happening because the negatively charged electrons moved from the cloud to the ground in an overall transfer of negative charges. However, “positive lighting strikes” do occur, they are just much rarer. Because they start at the top of the cloud instead of the base, the lightning strikes are much longer and as a result much more powerful, up to ten times stronger than negative lightning strikes.

Red sprites occur as stray ions collide with other molecules in the atmosphere. Photo: strangesounds.org

These positive lightning strikes are so powerful that they can rip apart molecules in the atmosphere into just ions. These ions are then capable of colliding with other molecules in the atmosphere like oxygen or hydrogen, which causes photons of red light to be emitted. These typically only happen in the upper atmosphere, but when they do happen they are called “red sprites”.

A massive blue jet, caused by positive ions charging upward into the sky. Photo: Phebe Pan

Another type of lightning can happen when massive amounts of positively charged ions rush upwards into the sky, trying to equalize the charge in the clouds. This phenomenon has been called “blue jets” and they can reach an impressive 40 kilometers high, though they too last for only a fraction of a second.

A Clap of Thunder

So how does lightning result from thunder? Thunder is created as the result of the rapid expansion of the air that surrounds a bolt of lightning. When a bolt of lightning occurs, a secondary lightning bolt will return from the ground to the clouds along the same path as the first bolt. This process happens extremely quickly, in only a few thousandths of a second, and the heat generated by this return stroke of lightning compresses the air around the lightning bolt.

The air has no time to expand since the lightning bolt occurs so quickly, meaning it is compressed to around 10 to 100 times normal atmospheric pressure. The air then rapidly expands outwards in every direction, creating a loud boom, in a process similar to the creation of a sonic boom. The sounds echo and reverberate through the air, creating the rumbling that people recognize as thunder. Thunder can be heard from 10-25 miles or less away from the lightning bolt that caused it.

Thunder follows a clap of lightning because the speed of the sound through the air is much slower than that of the electron flow that creates lightning. The sound reverberates outwards much like a shockwave, spreading out from the source. Meanwhile, the light from the flash reaches our eyeballs almost instantly.

It’s Still A Bit Of A Mystery

While we know a lot about what conditions are needed to create lightning, there’s still a lot that is unknown about how lightning is created. It is a complicated process, and there is much debate in the scientific community as to the exact process that creates lightning, such as how cloud gains an electrical charge in the first place.

One theory postulates that cosmic rays from space shoot down through clouds, and strip the electrons off of atoms as they move towards the Earth’s surface. These cosmic rays would drag the negatively charged particles toward the base of the cloud as they go, which would lead to the charge imbalance necessary to create lightning. However, this seems insufficient to create the massive ion imbalance in thunderstorms, so more research is being done into the process.

Lightning challenges much of our understanding of physics, but motivated by a desire to understand these processes, scientists continue to make progress in their research.

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