There is something nostalgic about sitting on a porch and watching a storm come in on a carefree summer’s day. Observing the clouds growing larger and blacker each minute, with the occasional flicker of lightning in the distance, I’d sit there and wonder about the storm. What are the bolts made of? Why do thunderstorms seem to be born out of distinct cloud formations? And how in the world is it possible for electricity to spring out of a cloud? This article aims to explain each of those questions, as well as, how the components of lightning are constructed out of atmospheric conditions, what exactly a lightning bolt looks like in slow motion, and lastly it will touch on more exotic forms of lightning.
To understand lightning we must first get a sense of how the right atmospheric conditions help to create a storm. Cumulonimbus clouds are the type that are massive in size, fluffy, vertical in shape, and close to the ground. They are often called mushroom heads and are associated with thunderstorms.
Because these clouds require a lot of energy to be created they are most frequent in the summer when the sun’s rays are at their strongest. The sun radiates its energy onto the surface of the Earth. This energy is absorbed and then radiated back in the atmosphere as heat.
As we know, hot air rises. An updraft carries water and gas molecules from lower in the atmosphere to higher and higher altitudes in the Troposphere, this process is called convection. As the water vapor rises farther it begins to cool down, this causes the vapor to condense into a water droplet. After enough material condense a cloud is formed.
[side note: have you ever wondered why it’s cooler after a thunderstorm? It is also due to convection. Just as hot air rises, cool air is pushed by downdrafts high within the cloud. Rain helps capture the coldness from inside a cloud, as well, bringing those of us on the surface of Earth, a much welcome relief from a hot summer’s day. Good ol’ convection!]
When water is in a vapor state and wants to condenses into a liquid state it must release some energy to do so; this energy is in the form of heat. The convection process works like a cycle, the heat released causes the moisture and air molecules to continue to rise within the cloud. The upwards draft pulls even more material into the cloud, growing ever more vertical until it hits the base of the Stratosphere. The cycle of convection within the cloud is called the hot air balloon effect.
When the cloud hits the edge between the first and second layer of atmosphere, it fans out due to the differences in air pressure and temperature between these layers. This is why so many cumulonimbus clouds have an anvil shape.
Each cloud column is called a cell and each cell lives for about a half hour before it’s energy runs out. The life time of a cumulonimbus is so short due the high amount of energy needed to create this type of cloud, thus it can not be sustained for long periods.
The primary reason why a cloud can conduct electricity is because of ions! It is both the ionization of water molecules and atmospheric gases that contribute to a lightning strike. These charged particles are incredibly interesting when you realize that without ions there would be no neon signs, no fluorescent lights, no auroras, no lightning and even no sun.
An ion is simply a particle or molecule that has either lost or gained an electron, leaving it with a net charge (if you need to brush up on the basic structure of an atoms check this out real quick).
Keep in mind there is never a transfer of positive charge like there is negative charge; protons never bounce from one atoms to another, only electrons can beshared between atoms.The one thing you need to remember is that a loss of electrons from an atom will give it a positive charge. A gain of electrons in an atom will give it a negative charge. To get a visual of this, click to enlarge the picture to the right.
Now let’s get back to the cloud. Once liquid water droplets have risen high enough, the water changes state once again, from liquid now into solid ice as it freezes. At high enough altitudes the updraft is no match for the weight of the ice particles, they now surrenders to gravity and begin to fall.
The interior of the cloud can be quite fierce. Updrafts can reach speeds of 15-30 miles per hour. It is due to this turbulent nature of the wind that sends ice molecules colliding into each other; in doing so they shed or gain electrons, becoming either an Anion (an ion with a negative charge) or an Cation (an ion with a positive charge). Some of the electrons shed during impact do not reconnect with other atoms, theses float around by themselves, they are called free electrons. (Electricity is made out of free electrons).
SEPARATION OF CHARGE: Ions with extra negative electrons collect at the base of the cloud and the positive ions are carried along with the updraft to fill the top of the cloud. The picture to the below is quite simplistic, however, you can get a visual of how the charges separate within the cloud. In reality it is never this cut and dry. Charges mix a bit more than this through out the cloud and it is possible to find pockets of cations in the base of the cloud along with the anions. But just to keep things simple imagine it looking like this:
PLASMA. The cloud is now primed to conduct electricity! This ionized cloud may now act as a plasma when agitated by an electrical current. Plasma is a distinct form of matter (as compared to solid, gas, or liquid). It is like a gas in that it has no defined shape and would distribute evenly within a container. However, unlike a gas, this matter it is made of ions and free electrons which have the ability to conduct electricity because they are charged. Plasmas are also very susceptible to magnetism, meaning the ions will follow along any magnetic field lines that may be near.
The large amount of negative charge at the base of the cloud causes the equilibrium of neutral charge between the cloud and the ground to break down. The surface of Earth has an overall neutral charge. It is mixed evenly with equal amounts of + and – charges. However, when something like a thundercloud is hovering overtop, the charges within the Earth begin to align according to the charge of the cloud. Meaning that the Earth can become positively charged due to electrostatic induction! In simple terms, this means that the charge of one material can affect the charge of another without even touching.
The electrons gathered in the cloud causes a repulsion of the electrons on the ground. They are pushed further down into the ground. Just as electrons are repelled by the cloud, positively charged ions are attracted to the cloud. This creates a static charge build up between the cloud and the surface of Earth.
THE BOLT BEGINS! Now that we have covered how ions and charges build up within the cloud and also within the ground, we can finally get to the really cool part, the lightning strike itself! When ever there is a static build up between two objects they yearn to meet and discharge their energy.
It is only because of the recent use of high-speed cameras and years of trying to point them in the right directions, that we know the anatomy of a lightning strike.
It begins with a small spark in the cloud. A stream of electrons flies out of from the base, it travels along a route to ground in about 50 yard segments; it then stops, pools for a couple billionths of a second and then splits and continues on path in a new direction toward the ground.
This initial bolt is called a stepped leader (SL). The picture of the house below shows this SL as it gets close the surface of Earth. This path of electrons is much fainter than what we would normally think of as a lightning bolt. Keep in mind there is still a build up of charge in the SL, it has not yet discharged any of its negative energy.
Notice the arrow coming out of the tree with little +’s leading to it. This is actually what is called a positive streamer (PS). This is a “stream” of positively charged ions that are attracted to the stepped leader, once again, due to electrostatic induction.
The PS exists for just a fraction of a second and is very rarely visible to the human eye. The length of a PS varies from just a few inches up to a few hundred feet above the ground.
The closest positive streamer to a particular stepped leader will become the most preferred route for electrical discharge. A SL might even bypass a taller object and connect with a PS closer to the ground. There are ways to predict the path of the stepped leader; they have to do with flux lines and, although, quite interesting it is not relevant to this discussion.
Positive streamers can reach out of anything on the surface of Earth, they can stretch out of blades of grass, tops of trees, roofs and even out of the top of your head. So if you are caught in a storm and your hair starts to get electrically charged, or you feel a tingling on your skin you better get the hell outta there, and fast, because you are now statically charged and a strike may be imminent.
The video below shows in slow motion detail how the stepped leader protrudes from the cloud and shows some great images of positive streams, as well.