I have a very intelligent three (almost four) year old who has a mixed relationship with thunder and lightning (see an older question from me, for example). He likes seeing it, but is also afraid of both. Part of this stems from probable sensory disorder - loud noises bother him much more than most children; but I think some of it is fear, as well (as he fears it even when it's not happening currently), in particular fear of the unknown.
One of the ways I'm trying to get over this, in addition to that from the other question, is to help him understand what lightning and thunder are in a physical sense - ie, an approachable but accurate scientific description. I want to help him understand what lightning and thunder are, what is dangerous about them, and what is not dangerous about them; and why we are (mostly) safe from both while indoors. In general, I explain physical phenomena as completely as possible, given his age; for example, I explain that rain comes from the sky when clouds are too full of water (which is true, to a limited extent). I won't explain barometric pressure yet, but cold/hot air masses I might explain at some point soon.
How can I explain this to a preschooler, so that he will have as complete of an understanding as possible at his age, and in particular focusing on having a good idea (in his head) of what they are so they're not as scary?