system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 This might be a dumb question but it just started raining, so I started thinking, why does it start raining slow then increase in speed? Like what is happening in the cloud that's different than before, or vice versa if it starts pouring like there's no tomorrow then slows down?
gpt
The cloud moves.

First, the cloud's (thinner) edge comes over you, and because it's thinner there's less rain to unload; then the bulk of the cloud, then finally the trailing edge.

Clouds are water vapor suspended in air (and so is fog), and the ability of air to keep water depends on temperature and pressure.  Air is always moving up there, and at some point the air that contains a cloud may hit colder air or different pressure air, and the cloud will start unloading.  

But it keeps moving; air keeps moving it along.

So, more rain vs. less rain depends on the thickness of the part of the cloud that happens to be passing above you at any point in time.