Rain's dirty little secret

Somewhere inside of every raindrop is a tiny impurity – a touch of salt, a speck of soot, a grain of clay – that’s absolutely crucial to the raindrop’s existence. In fact, without these microscopic pieces of dirt, there would be no rain, because water vapor can’t condense into droplets on its own. Which is kind of weird, because water molecules like each other – if they didn’t, they wouldn’t cling to each other like this! And in the air, vaporized water molecules collide and stick together all the time – but they also break apart all the time, thanks to bond-breaking heat energy. 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. But actually, that’s only true if the cluster is big to start with. If it’s too small, its surface is so curved that the molecules on the outside have few neighbors to bond to, which makes them easy to break off, so the cluster as a whole has higher chances of losing molecules than gaining them, even below the dew point. Which means that up until that critical size, a cluster’s odds of shrinking are much better than its odds of growing. Unfortunately, that critical size is 150 million molecules. And while there are millions of five-molecule clusters in a golf-ball sized volume of air at dew point, odds are that only one of those clusters will grow to a size of ten. And you’d need a golf ball of air ten million miles across to find one 50-molecule cluster! Which, basically, means that clusters of water molecules never get to 150 million on their own. Fortunately, they don’t have to – they can start off at that critical size by condensing onto one of the gadjillions of little pieces of dirt floating in our atmosphere, and then grow and grow until they’re a droplet in a raincloud. So, ultimately, it’s these little pieces of dirt, surrounded by water, that make life possible on our big piece of dirt surrounded by water.

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