Scientists who study clouds and water droplet formation help us understand why rain never pours from the clouds in a stream but falls in drops.

Clouds are simply very small particles of water. They stay suspended because the force of gravity pulling them down is less than the resistance of the air through which the raindrop has to move.

Raindrops start when water condenses on a tiny dust particle. These invisible droplets soon attract enough extra water to form a visible drop. The smallest drops we call drizzle. They hardly seem to fall at all – travelling only at about one kilometre an hour. In comparison, during a tropical downpour, raindrops can get as large as six millimetres. They can fall at about thirty kilometres an hour, before breaking up again into smaller drops because of air resistance.

There are several forces at work when raindrops develop. A force called surface tension causes water to stick together and form little balls. When the balls are big enough, gravity pulls them down as raindrops. As they fall, the raindrops gradually grow by colliding with other water drops. As they grow, they accelerate under the influence of gravity. But frictional and aerodynamic forces are also at work on the drops to limit their size.

So next time you’re caught in a storm, be thankful that the balance of forces favours drops rather than streams when rain falls from the sky.

Bob