The anatomy of a lightning stroke is surprisingly complex. For lightning to occur, of course, there needs to be a source of electricity: an electrical storm. Thunderstorms contain clouds that work like capacitors: the tops of the clouds have a positive charge and the bottoms are negatively charged. Scientists believe that this electrical charge may build up as the result of electrons being exchanged by particles within the clouds as they collide with one another due to temperature differences within the cloud, though just how this happens is a matter of debate. As the difference between the positively charged cloud tops and negatively charged bottoms increases, an electric field is generated.

As the charge within the cloud builds, it has an effect on the surface of the Earth directly below, as well as objects on top of the ground. The negative field on the bottoms of the clouds repels electrons on the ground, which in turn causes the land below the storm to become positively charged. As the charge builds and builds, the air between Earth and cloud becomes ionized—air molecules break down into electrons and positively charged ions—transitioning into a plasma state. This plasma now serves as a conductor between the clouds and the ground below (or to other clouds, or into the surrounding air).

Next, the cloud sends out “stepped leaders,” which are precursors to the oncoming flash of lightning. Think of them as ionized paths and streams of electrons that are like feelers that the cloud sends out, searching for the best path for the lighting bolt to flow through. Sometimes they may be visible as faintly glowing purplish streaks.

Meanwhile, as the stepped leaders are sending exploring fingers outward, the positively charged ground or other receiving object is sending out feelers upwards. These feelers, called “positive streamers,” don’t always connect with the stepped leaders, but when they do the circuit is completed and the fireworks are ready to begin. Positive streamers can emerge from any inanimate or living object, including people.

Like railway workers completing a railroad, once the two ends of the path join, the train can drive through. This is the lightning bolt, an explosion of energy that occurs as Nature attempts to equalize the charge between cloud and surface.