Listen to part of a lecture in an Earth science class.

Lightning is one of the most common meteorological events on Earth. It occurs somewhere on the planet roughly 40 times a second. Although the most common type of lightning occurs between clouds, the type of lightning we are most familiar with is so-called cloud-to-ground lightning, the kind we see during thunderstorms. And that's what I want to focus on today.

Despite its frequency, lightning is difficult to study, since the process only takes around half a second from beginning to end. But here's the generally accepted scenario of lightning formation.

Now, a storm cloud is a very turbulent environment, with powerful updrafts and downdrafts occurring continuously. And these air movements carry drops of water and ice particles that are constantly colliding. This causes negatively charged electrons to be released from the rising water particles and to collect on the solid falling particles.

So we end up with a positive charge at the top of the cloud and a negative charge at the bottom. At the same time, this negative charge at the bottom of the cloud creates a corresponding shadow of positive charge on the surface of the Earth below it.

Now, the atmosphere between the cloud and the Earth's surface does not conduct electricity very well. It's a very good insulator. This means that it normally prevents a connection and electrical circuit from forming between the two.

But at some point, the cloud's charge becomes too great for the air to restrain it. So much energy builds up within the cloud itself that the negative charge at the bottom eventually becomes much greater than the ground's positive charge. The electrical imbalance between the two reaches a tipping point, and the resulting lightning flash is an attempt to equalize them.

So in cloud-to-ground lightning, a pathway forms through the atmosphere. This pathway is called a leader, because it leads a trail of negative charges from the cloud to the ground below. And the leader progresses downward in short steps, maybe 50 meters at a time.

Now, each step doesn't go in exactly the same direction. It's not predictable, but the steps are generally downward, attracted by the positive charge on the Earth's surface, directly below the cloud. This is what gives lightning its characteristic, jagged, forked appearance.

Now, once the leader reaches the ground, the circuit between the negative charge in the cloud and the positive charge on the ground is complete. But interestingly enough, we don't actually see this leader as it forges a pathway down through the atmosphere. The lightning bolt that we do see is a powerful bolt of positive electricity called the return stroke.

Now, this return stroke leaps back up the pathway from the ground. It distributes a positive charge to the cloud, balancing the electrical field. The energy in this return stroke can be intense. It's hotter than the surface of the sun. Frequently, after the return stroke, there will be another leader that will recharge the same pathway, which leads to another return stroke. And this fast succession of leaders and return strokes is what causes lightning to flicker. And all this happens, as I said, in about half a second.

Now, I hope you noticed I said that the lightning we do see is going from the ground to the cloud. I know when we see lightning, it seems to be going the other way, from the cloud to the ground, and it also seems to last longer than one second. But both of those observations are untrue. Remember that human eyes are really not the best scientific instruments, especially when it comes to things that only last a fraction of a second.

Now, because the process of electric buildup and discharging clouds is so quick, it's virtually impossible to detect to find evidence of it, which is why I said earlier that what I'm describing is the generally accepted explanation.