system
Given the following post, respond as if you're commenting on Reddit:
human
This may seem like a silly question, but how/why does lightning occur?

I know it sounds ridiculous for a 19 year old to be asking this, but I was just staring at the sky and I realized that I have no idea why lightning occurs. Rain and the water cycle make sense to me, but lightning...
gpt
Lightning occurs due to large regions of positive and negative charge that build up within a thundercloud.  When enough separated charge builds up the lightning process begins, culminating in the large discharge that we see and/or hear.  Most lightning (on the order of 80%) occurs between two clouds or within the same cloud, only a fraction of lightning reaches ground.

Now you may ask, why do charges separate in thunderclouds?  Generally convective air (the cycle of air going up and then back down) carries with it a lot of water, as the up going air crosses the freezing point in a cloud it starts to freeze.  So on the way back down it is mostly ice.  When the upwelling water collides with the downwelling ice there is a small exchange of electric charge between the particles: negative charge goes up, positive charge goes down.  This results in the large charge separation in thunderstorms (on the order of 20 megavolts).

Without the large convective regions lightning will not occur.  So relatively weak convection on the West Coast of the United States results in little lightning while strong convection over the great plains results in massive thunderstorms.