I bet you know the basics. Water vapor condenses into drops that form clouds, then rain. Yeah, but how do the tiny droplets of the cloud coalesce into drops large enough to fall as rain? I bet you didn’t know that we didn’t know.
Now we have a good idea: turbulence at tiny scales.
Nerdy details here: Physicists Solve Cloud Formation Puzzle – Technology Review
Not that it will help Colombia. See previous post.