Why do clouds go grey when it's about to rain?

When fluffy white clouds turn heavy and grey, you know rain is on the way. But why are rain clouds grey?

Bouncy light

That’s because there’s always a battle going on between sunshine and clouds. Clouds are made up of billions of water droplets and ice crystals. Any rays of sunshine that try to get through this wall of water end up colliding with the droplets, and the light bounces off in all directions. Every time light hits a droplet, it loses some of its brightness.

When a cloud is thin, any light that passes through it is still nice and strong after its battle with the cloud, so that cloud will look bright white.

Full of water

But the bigger and denser a cloud is, the more water it holds, and the more light it can stop. That’s why the cloud looks darker.

It makes sense that when a cloud is full of water, it’s more likely to drop that water on us as rain. So now you know why rain clouds look grey.

Answered by Sebastiaan van de Water