by David Murphy
Lightning occurs in thunderstorms and while there are various, complicated descriptions as to the physics behind it, here is one, simplified explanation that I picked up in meteorology school.
Lightning forms when different electrical charges (some positive, some negative) become separated in the storm's turbulent currents and are then drawn back together. Have you ever heard the phrase, "Opposites attract"? While some use these words to describe what may or may not be a true aspect to human relationships, the phrase actually comes from the world of physics in which particles with opposite electrical charges are naturally drawn together. That's exactly what's happening with lightning.
In a thunderstorm, raindrops are drawn high into the storm's upper region by its updraft, a column of rapidly-rising air. The drops are swept so high, they freeze. The bits of ice, still caught in the violent updraft, are sent smashing into each other, so that they shatter into smaller bits. The lighter, outer pieces, which carry a positive electrical charge, are blown up to the top of the storm, while the larger centers of the ice chunks, which carry a negative charge, remain in the middle of the storm, too heavy to be carried farther upward by the wind. Meanwhile, the ground beneath the storm adopts a positive charge.
Picturing this for a moment, we now have a scenario where three separate layers of differing charges exist: positive at the surface, negative in the lower portion of the storm cloud and another positively charged layer in the top of the cloud.
Eventually, as the storm evolves and the turbulence continues, these opposite charges are in some cases thrown back closer to each other; so close, in fact, that they can no longer resist being separated. The lightning bolt is the electrical link that reunites these opposite charges and restores order in the disjointed physics of the storm cloud. You can think of the bolt as a sort of highway along which these particles travel on their way to reconnecting with their lost opposites. Of course, it's a pretty violent reunion, as the lightning bolt forms and fuses these particles back together in a super-heated split second.