The sizes of raindrops are caused by the break up of individual droplets, reports a study published online this week in Nature Physics. This is a much simpler mechanism than was previously thought.

When raindrops hit the ground they do so in a wide range of sizes. It was thought that this size distribution was the result of a complex series of interactions between the droplets as they fall. But by analyzing high-speed movies of falling water droplets, Emmanuel Villermaux and Benjamin Bossa show that this distribution is caused by the fragmentation of individual, non-interacting raindrops.

The movies demonstrate that as an initially spherical droplet falls, it gradually flattens out into a pancake shape. As it gets wider and thinner, it eventually captures the air in front of it to form the shape of an upturned bag. Finally, as the bag inflates to a certain size, it breaks apart into many smaller droplets ― droplets whose size distribution mimics that of natural rainfall.