The sky is blue because air bounces blue light everywhere. So why aren't clouds blue too?
1What you need to know
Sunlight is all the colours, and the bouncing depends on SIZE
Two ideas, then the experiment.
βοΈ Sunlight = all colours mixed
Sunlight looks white, but it's actually all the colours packed together β red, orange, yellow, green, blue, violet. A prism can split them apart.
π― 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 β but a very different size.
Air molecule
~0.3 nm wide bounces blue
Cloud droplet
~10β100 Β΅m wide β¦what does it bounce?
Air molecules are about 0.3 nanometres wide. Cloud droplets are 10β100 micrometres β that's tens of thousands of times wider. Does a giant still pick blue?
3The experiment
Predict first β then grow it
The speck starts at air-molecule size (watch the blue rays). But what happens if it grows all the way to cloud-droplet size?
You slowly grow the speck from air-molecule-sized to cloud-droplet-sized. What colour does the bounced light become?
Lock in your guess β the slider unlocks after.
Your guess is locked in β now drag the speck all the way:
Speck size: air molecule
air moleculecloud droplet
Drag all the way right β watch the scattered rays closely.
5But waitβ¦
It's not quite that simple
If clouds bounce all colours, why do some look grey? οΌ
The droplets inside a storm cloud are still bouncing all colours equally β 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? οΌ
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 β blue sky. Giant cloud droplets don't care which colour β they bounce them all equally, and every colour together is white.
psst, grown-ups β οΌ
Rayleigh vs Mie scattering. Air molecules (~0.3 nm) are far smaller than visible wavelengths, so scattering follows Rayleigh's law: intensity β 1/Ξ»β΄, strongly favouring short (blue) wavelengths. Cloud droplets (~10β100 Β΅m) are far larger than visible wavelengths β scattering enters the Mie regime, which is nearly wavelength-independent, so all colours scatter equally and the cloud looks white. 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? οΌ
The droplets are still bouncing all colours β 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? οΌ
Rainbows need big falling water drops β 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? οΌ
At sunset the sunlight arriving at the cloud is already orange-red β 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.