音频下载[点击右键另存为]
When you get caught in a downpour, you probably don’t think about the size of the raindrops that assault you as you run for cover. But physicists do. And they’ve come to the conclusion that the drops that hit the ground, or your head, are the shattered remains of bigger drops that fell from the clouds.
Raindrops come in a variety of sizes, even within the same storm. And scientists used to think that, to get that kind of distribution, raindrops must crash into each other on the way down, breaking up into smaller droplets or coalescing into larger ones. Now a team of French scientists has produced high-speed footage of falling water droplets. And they find that drops of different dimensions don’t require collision—they come from the fragmentation of individual, isolated droplets. Their results appear online in the journal Nature Physics.*
The video evidence reveals that water droplets first flatten out as they fall. And as these plummeting pancakes get wider and thinner they eventually capture air, forming what look like little plastic grocery bags floating in a breeze. And when the bags get big enough, they pop. And you’re left wondering why you can never remember your umbrella.
—Karen Hopkin