The Causes of Lightning:
Lightning occurs because of the collision of cloud droplets inside a thunderstorm. Initially, these droplets are neutrally charged and do not electromagnetically interact with each other. However, when these droplets collide, electrons are scraped off the water molecules. As a result, these water molecules are now ions with a net positive charge and rise toward the top of the cloud. The electrons tend to gather toward the base of the cloud
It’s not only these collisions that are responsible for creating a giant capacitor of sorts within the cloud. As cloud droplets rise into the upper atmosphere, they cool. But just because they cool below the freezing point does not mean they immediately turn into ice. Some droplets are supercooled, meaning they are liquid at temperatures below freezing. The ice crystals that form are negatively charged, and the supercooled water droplets are positively charged. Updrafts separate the frozen and unfrozen droplets and take the supercooled, positively-charged droplets up to the top of the cloud, while the frozen, negatively-charged droplets travel to the lower portions of the cloud.
With this charge separation comes an electric field which, like the cloud, is generally negative at the base and positive at the top. This field becomes stronger and stronger as the charges in the cloud become stronger and stronger. In fact, the field can be so strong that the electrons at the base of the cloud can repel the electrons on the surface of Earth into the ground while attracting positive ions upward. Now, you have an “electron sandwich,” with positive charges enclosing a region of negative charge. At the very base of the cloud, there is a weak net positive charge, but for the purposes of simplicity, don’t give it too much thought. Bottom = negative, top = positive.
For lightning to occur, the atmosphere has to be enough of a conductor (like metal) so that charges can move charges and align themselves in the above position, and enough of an insulator (like rubber) so that charges can build up without being constantly transferred from one place to each other. For lightning to form, all we need is for the charge separation to become so great that a conductive path is formed through the air and provides a line through which the charges can freely interact and release excess charge. But how do we form that electric path?
Air Ionization and Plasma:
If the electric field becomes exceptionally strong (tens of thousands of volts per inch), the air itself is separated into electrons and positive ions. When this happens, the air becomes a much better conductor of electricity and is called plasma. This process of separation lays out an easy path for electricity to travel from higher potential to lower potential.
Even though plasma is an excellent conductor of electricity, we don’t see a constant exchange of electrons and positive ions from one place to another. This is because there is not a homogeneous plasma field that would allow this. Instead, we have step leaders, which are independent paths of ionized air that stem from the negatively-charged region of the cloud. This happens with all types of lightning, but I think it is best visualized using cloud-to-ground lightning, so let’s explain it that way.
These leaders come in all different shapes and sizes and the atmosphere is filled with particulate matter that can make the leader more likely to go in one direction than the other. If the base of the cloud and the ground are parallel, the electric flux, which is a scalar quantity that represents the rate of flow of the electric field through an area (and therefore the strength of the field), between the cloud and the ground will be maximized when the area through which the flux is being measured is parallel to the ground. By approximating the direction and magnitude of the flux by using arbitrary ‘flux lines,’ we can see this since flux lines always radiate perpendicularly from their charge surface and then move in the direction of opposite charge.
Of course, no cloud can be perfectly parallel to the ground. The cloud and ground would have to be to identical and parallel planes, and such an idealized situation is not found in nature. And then there’s the whole “the Earth isn’t flat” thing, not to mention all the particles in and characteristics of the atmosphere that would interfere with the paths of the flux lines. As such,the flux lines will not follow a path straight from the cloud into the ground. Instead, these flux lines intersect and diverge, creating a non-uniform field. It is this non-uniform field that causes the step leaders to take a path not perpendicular to the surface of the intended target.
As you can see below, there is a weak amount of positive charge at the very base of the cloud. As you can see above, I told you to not worry about it to make things easier on yourself. This charge is not sufficient to neutralize the large buildup of static electricity and the electrons continue to flow to the ground.
These leaders occur in stages. It may not look like there is a pause between them, but that is just because they are occurring so fast. They develop downward on either side of 200,000 mph, but they would be going closer to 186,000 miles per second (the speed of light in a vacuum) if they went downward in an unfragmented fashion.
Now, you’ve got leaders going toward the ground via the path of least resistance. These leaders are slightly purple. New leaders may form, but every one stays illuminated until the current has reached the ground.
These leaders are NOT the big lightning strikes we are familiar with. In order for those to occur, the circuit needs to be completed. And that’s where positive streamers come in.
The whole leaders/streamers thing is very similar to a handshake. As public image consultant Álvaro Gordoa shows, somebody always initiates a handshake, and the person who picks up this signal immediately knows to reach their hand out and complete the gesture. The hand of the initiatior doesn’t just keep on truckin’ until it smashes into the closed fist of the now unsettled would-be recipient of the handshake. And that’s what happens with lightning. The stepped leader doesn’t just smash into the ground. It meets with what is called a positive streamer to complete the transfer of electrons from cloud to ground.
Since these leaders are negatively charged and the ground is positively charged, the charges attract each other. The ground manifests this attraction by sending these streamers, also purplish in color, into the atmosphere. Once they have been produced, they do not travel up to meet the leaders; they let the leaders come to them. In other words, the hand receiving the handshake from the initiator lets the initiating hand to all the work.
Once these two acquaintances have met, a path for the current to flow between the ground and the cloud is created, and a huge discharge follows. Positive charge from the ground races up this path toward the thundercloud and is visible as the “return stroke” of lightning that is most visible to us. That’s right: the strike we see with our naked eye actually starts on the ground and races back to the cloud. Negative charge does all the hard work trying to find a way to get to the ground, and once a connection is established, positive charge surges back up through the circuit.
There can be as many as 30-40 additional strikes after the initial return stroke. Remember how the step leaders coming to the ground from the base of the cloud were pointed in all these different directions? Well, once the circuit is completed, the electrons in those leaders flow through the leaders into the path of the initial strike. The leaders are only providing a path for the electrons to flow through, not neutralizing the charges themselves. These secondary strikes can be seen as branches off of the initial strike if they follow different paths or just make the initial strike look longer by taking the same pathway (and anywhere in between). Sometimes, the flash from the main strike will end while secondary strikes are occurring, and this makes the initial lightning strike flicker like a star or blind you like a strobe depending on your proximity to it.
The return stroke, which can be thought of as a plasma channel for charge to travel through, discharges a tremendous amount of static electricity in a very short period of time and therefore heats the air around it to extremely high temperatures – as high as 50,000 degrees C. This heating causes air radiating from the return stroke to expand in the form of a shock wave that we hear as thunder. Thunder is LOUD… my house just barely missed getting struck by lightning during ‘thundersnow’ storm (a thunderstorm where it is raining instead of snowing) on December 18, 2008. The family cat has never been the same since.
Paths of Lightning:
The example we used above was of cloud-to-ground lightning, which is the most dangerous. People, trees, and animals don’t live in clouds now, do they? There are two other paths that lightning can take: intra-cloud and cloud to cloud. Remember, it’s not just the base of the cloud and the ground that are charged. There’s a whole bunch of positive charge at the tops of the clouds, and lightning can form either in the same cloud or spread from different clouds due to the interactions between these charges. The process through which the lightning takes these paths has all the plasma and streamers and associated phenomena that we went over in cloud-to-ground lightning. Cloud-to-ground lightning is just easier to visualize.
To leave you with something to ponder, do yourself a favor and look up “ball lightning.” It’s a truly fascinating phenomenon.
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Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017