There is little theory left regarding lightning and thunder. There are still things we don’t know about lightning, however we do know what causes both lightning and thunder. It is still worthwhile to gain a greater understanding and appreciation for this weather phenomenon, since it is so common.
Lightening is a form of static electricity, much like what happens when a person shuffles over a thick carpet and touches a grounded metallic object. However, it is also extremely more powerful. A lightening bolt is usually generating thousands of volts of electricity. Like all electromagnetic energy, this also produces heat as a byproduct. Because of the voltage, this is also often extreme.
As with the minor example of static electricity above, involving shuffling across the carpet, the cause is the difference in polarity between two objects. That is, if one object ends up with a positive charge, and another has a negative charge, a discharge often occurs in order to equalize the difference between the two.
Friction is often the reason for an object, such as a cloud, picking up an electrical charge. An easy experiment to show this is to take a balloon, rub it against your hair, and place it against a wall. Rubbing the balloon against your hair allows it to pick up a charge, which is the reason it will stick to the wall, which is usually electrically neutral or nearly so.
In a cloud, there is friction with the atmosphere, with dust and dirt particles, and even with droplets of water. This means that one cloud can form a positive charge, and another can form a negative charge, simply because of differences in the root of the friction. This can lead to cloud-to-cloud lightning, such as sheet lightning.
Clouds can also end up with a charge that is the opposite of the one found on the ground. This results in lightning strikes that meet the ground. Most of the forest fires in the western US are caused by this form of discharge.
The interesting part is that a lightning bolt is made up of more than one bolt, though we may only see one, and the bolt actually doesn’t travel from the cloud to the ground, as we see it. At least, it doesn’t do so entirely.
The sequence is extremely fast. A charged cloud attracts extremely small and dim streamers from the ground or objects like trees and buildings that are ‘grounded’. This produces a conduit for a discharge from the cloud.
What we see as the flash of lightning is the discharge. Since the streamers or filaments coming from the ground are deflected by both particles and wind currents, the flash is also usually in a zigzag pattern, as the discharge follows the conduit to the ground.
Interestingly, though only around an inch across, the streamers can and have been photographed with cameras having very fast shutter speeds. Even more interesting, a bolt of lightning that strikes the ground or an object on it is an average of four inches across, though it may appear much larger because of the brilliance of the flash. This is actually an optical illusion, though there are exceptions.
The sound of thunder is the bow shock wave of the passage of the lightning bolt. A person can see this in a much more minor way by looking at the passage of a ship through water, from above. In front of the ship, as it cuts through the water, the fluid is piled up and flows to either side of the bow. This is the bow wave.
Though tremendously faster, the same thing happens when a bolt of lightning slices through the air. Superheated, the air immediately around the bolt expands, creating a vacuum, while at the same time; a bow wave is created in the air. The force of the bow wave, and the air rushing in to fill the vacuum caused by the lightning, combine to create the crack and rumble of thunder.
Much the same happens when a jet goes supersonic. The bow wave spreads out in the form of sound waves we hear. The sound is the waves that are deflected around the object, such as the ship or jet.
We still don’t fully understand the mechanism for ball lightning, nor do we know all there is to know about lightning, however there is no longer a theory to work with. It has been proven in many aspects.
We know what it is, what the mechanism is, what causes it, and what the thunder afterward is caused by. For many decades, we’ve even had and used the means to protect buildings against damage that could happen because of lightning strikes.
Lightning and thunder have transcended the realm of theory, mystery, and superstition. There is still much to learn about this fascinating weather phenomenon.