In the nineteenth centuary scientists studying the weather looked at raindrop sizes, and discovered something quite interesting. There was quite a variation, most were less than 1mm across but a few were up to 5mm across, but for the same rate of rainfall the distribution was always the same.
This is strange as there are lots of ways in which rain can be created, at different heights and rates, depending on the clouds it is coming from. At the time it was thought that it was because big drops were crashing into one another and breaking apart. The problem is that there just aren't enough raindrops in the sky to crash into each other so scientists have remained confused.
Emmanuel Villermaux at Aix-Marseille University has probably worked out what is going on. There is a well known effect on fast moving droplets of diesel in an engine, where they flatten, then blow up to form a parachute shape and then pop to form smaller droplets.
This is an effect I noticed on a high speed video of our water rockets kitchen science experiment. right at the end of this video, watch the last water droplet falling.
So they did some experiments and discovered that the distribution of droplets coming from one of these bubbles is exactly what you find in raindrops, so this may solve the century old mystery.
They have yet to see this process occurring with raindrops, put they are flying high speed cameras on planes looking for the events.