Yesterday, Henry Reich explained in the latest episode of MinutePhysics that, according to the law of physics, rain shouldn't actually exist, because water vapour can't actually condense into water droplets on its own. Without the microscopic particles of dirt, dust, soot and clay that get trapped in rain droplets, the water molecules struggle to stay together strongly enough to form droplets.
Which is weird, says Emily Elert in the latest episode of MinuteEarth above, because water molecules actually really like each other. In the air, vapourised water molecules collide and stick to each other all the time, she says, but heat energy has a tendency to break them apart. "Only when the air cools down past a certain point - called the dew point - does this breaking apart slow down enough for little clusters of water molecules to grow into droplets," says Emily.
But it's actually not that simple, because this only works if the cluster of water molecules is big to start with, because a small water droplet has a rounder surface than a large one, which means there are less neighbouring molecules to bond with, and the cluster is more likely to lose molecules than to gain them, explains the video above.
Scientists have figured out that because of this, up to a critical size - 150 million molecules - a water droplet's chances of shrinking are greater than the odds of it growing. And, as Emily explains so well in the video above, it's physically impossible for that many water molecules to accumulate. Unless they're surrounded by dirt particles.
I'll let the episode of MinuteEarth above explain it to you, because it looks so good when they do. And remember to watch the latest episode of MinutePhysics too, which explains more about the physics of why raindrops shouldn't really exist.