Lazy Lightning

Why does lightning usually strike the highest point?

We can only begin by talking about what exactly lightning is. But to talk about lightning, we'll have to start with electricity. Electricity is a form of energy that's simply the movement of electrons. For instance, the current through the wires in your house consists of the flow of electrons. If you take a closer look, you'll notice that the wires are usually all made of copper or aluminum, or more commonly: metals. And what are metals? These are elements that can easily conduct electricity, by facilitating the movement of electrons through it. More specifically, the outer electrons in metals are shared between different atoms. Simply put, a sea of electrons move about and serve as the outer electrons for many metal atoms (Figure 1). These electrons are not bound tightly and are "free" to move about. That's why electricity conducts well through metal atoms; The electrons can flow through it easily.

Figure 1

Now that we have a basic understanding of electricity and conductors, let's return to our story in the sky. On a stormy night, strong updrafts and downdrafts in the wind meet each other. The wind that's elevated through the updrafts contains small water droplets. Meanwhile, the air coming through the downdrafts brings hail and ice from the frozen upper regions of the storm. The turbulent conditions of the storm cause the updraft and downdraft to collide. In these collisions, the water droplets from the updraft transfer heat to the hail and ice from the updraft: the result is that the water droplets freeze and the frozen hail softens. As this soft hail collides with other water droplets and ice particles, electrons are shaved off the particles in the updraft. The electrons collect at the bottom of the cloud and the bottom of the charge is left with a negative charge (Figure 2).

However, there's an issue. The air in the atmosphere around the clouds consists mostly of nitrogen, oxygen, and carbon dioxide gas. Unlike metals, these elements are tightly bound to their electrons, preventing the flow of electricity through them. Stuck in the cloud, the negative charge accumulates more and more as the strength of its electric field grows stronger. Eventually, the build-up of all this negative charge will be released in what we see as lightning (Figure 4).

Figure 2

Figure 3

Moments before the lightning event, the strong electric field of the negatively charged part of the cloud has attracted positively charged ions on the ground. Positive charges accumulate and are pulled towards the cloud, eventually getting pulled to higher points as they attempt to get closer to the negative charges in the cloud. So if the positive charges accumulate at higher points, why doesn't lighting always hit the highest possible point? The place where the most positive charges accumulate also depends on the conductivity of the material, which is why lightning rods (Figure 3) at the top of buildings are often copper. The electric field continues to grow stronger as more electrons accumulate at the bottom of the cloud and the pool of positive charges on the ground too becomes larger. Eventually, the electrical attraction between the two is so strong that the two are pulled together.

Though the air in the atmosphere does not conduct electricity well, the great strength of the electric field rips electrons off the atoms of nitrogen, oxygen, and carbon dioxide gas as it makes its way to the pool of positive charges. This process is what we see as lightning. The lightning may seem "lazy" for merely striking what seems to be the closest point to the sky, but the true reason lies in the collection of positive charges.

Figure 4