Book 7, Question 8
Why Does Rain Fall as Drops, Not One Big Splash?
Inside a cloud there are millions of tiny droplets that are too light to fall. They bump into each other and join up, growing bigger and bigger. Once a drop is heavy enough, it falls, and every drop travels on its own.
More fun facts
- Water vapor first turns into tiny cloud droplets around specks of dust, salt or smoke.
- Because those specks come in different sizes, the cloud droplets do too.
- The biggest droplets fall faster than the small ones.
- As the big droplets fall, they crash into smaller ones and often stick together. Scientists call this collision and coalescence.
- A cloud must be thick enough for droplets to have time to grow big enough to fall as rain.
- A NOAA classroom activity uses ping pong balls in a stream of air to show how droplets bump together.
Questions kids ask
Why don't clouds rain all the time?
Cloud droplets have to bump together and grow big enough before they can fall as rain.
What is coalescence?
When falling droplets crash into each other and join into one bigger drop.