Why are clouds white?

1What you need to know

Sunlight is all the colours, and the bouncing depends on SIZE

Two ideas, then the experiment.

☀️ Sunlight = all colours mixed

Sunlight looks white, but it's actually all the colours packed together — red, orange, yellow, green, blue, violet. A prism can split them apart.

🎯 Tiny things love blue

When sunlight hits a tiny air speck, the speck bounces blue light far more than red. The size of the speck is the secret. But what happens when the speck gets bigger?

2Meet the two specks

Air molecule vs cloud droplet

Same sunlight, same sky — but a very different size.

Air molecule

~0.3 nm wide
bounces blue

Cloud droplet

~10–100 µm wide
…what does it bounce?

Air molecules are about 0.3 nanometres wide. Cloud droplets are 10–100 micrometres — that's tens of thousands of times wider. Does a giant still pick blue?

3The experiment

A new case: change the sunlight

Cloud droplets scatter visible colors much more evenly than air molecules. Keep this droplet the same size. Now change only the light arriving at it.

At sunset, only orange-red light reaches this cloud droplet.
Will the scattered light stay white, turn orange-red, or disappear?

Lock in your guess — the slider unlocks after.

5But wait…

It's not quite that simple

If clouds bounce all colours, why do some look grey?

The droplets inside a storm cloud are still bouncing all colours equally — but the cloud is so thick that most of the light bounces around so many times inside it that very little makes it out to your eyes. Less light reaching you = grey or dark. The thicker the cloud, the darker the base.

So why doesn't the whole sky turn white?

The air is mostly empty space. The tiny air molecules are so spread out that most sunlight passes straight through and only the blue builds up across the whole sky. It takes a packed crowd of cloud droplets to bounce enough light to look white.

It's all about size. Tiny air specks love blue and bounce it everywhere — blue sky. Giant cloud droplets don't care which colour — they scatter the colors arriving at them more evenly. White sunlight makes a white cloud; orange-red light can make an orange-red cloud.

psst, grown-ups →

Rayleigh vs Mie scattering. Air molecules (~0.3 nm) are far smaller than visible wavelengths, so scattering follows Rayleigh's law: intensity ∝ 1/λ⁴, strongly favouring short (blue) wavelengths. Cloud droplets (~10–100 µm) are far larger than visible wavelengths — scattering enters the Mie regime, which is nearly wavelength-independent, so visible colors scatter more evenly. The cloud color also depends on the illumination: white light can look white, while orange-red illumination can look orange-red. Very thick clouds attenuate most light through multiple scattering, making their bases appear grey.

What else makes you wonder?

Why do storm clouds look so dark and scary?

The droplets are still bouncing all colours — but the cloud is so thick that almost no light can make it all the way through or back out. Imagine trying to see through a wall of bouncing balls that never let you peek through.

How does a rainbow form if clouds just make white?

Rainbows need big falling water drops — not the tiny floating cloud droplets. Big drops act like tiny prisms, bending each colour by a slightly different angle and sorting them into a ring of colour in the sky.

Why do clouds turn orange and pink at sunset?

At sunset the sunlight arriving at the cloud is already orange-red — the blue was scattered away by all the extra air it had to cross. The cloud then bounces that orange light equally in all directions, so the whole cloud glows orange or pink.