system
Given the following post, respond as if you're commenting on Reddit:
human
How do clouds generate the electrical energy needed for lightning strikes?
gpt
Interactions between colliding precipitation particles cause them to transfer charge.  Thunderstorm clouds have two main forms of solid particles: tiny ice crystals, and larger graupel.  When ice hits graupel, ice tends to pick up a positive charge, while graupel picks up a negative charge.

Now, here's the cool part.  Ice crystals are tiny and get carried around by wind really easily.  Thunderstorms require updrafts to form and survive, so the ice gets carried upward.  Graupel is big and dense enough that it falls through the wind.  Meaning that the positive ice moves up and gives the upper cloud a positive charge, while the negative graupel moves down and gives the lower cloud a negative charge.

The ground is a good conductor of electricity, so when the lower cloud develops a negative charge, it induces a positive charge in the ground below the cloud.  So, there are three charged regions: the positive upper cloud, the negative lower cloud, and the positive ground.

So, intra-cloud lightning can occur between the top and bottom of the cloud.  Cloud-ground lightning normally carries negative charge from the lower cloud to the ground.  But, sometimes the upper cloud can form a strike that goes all the way around the lower cloud and hits the ground; this transfers positive charge to the ground, and is often called a "bolt from the blue" because it goes around the outside of the cloud to places that can have clear skies.