If the sky's blue comes from bouncing light, why aren't clouds blue too?
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After you watchIf the sky's blue comes from bouncing light, why aren't clouds blue too?
The short answer
Clouds are white because their water droplets are enormous compared to air molecules — air molecules are about 0.3 nanometres wide; cloud droplets are 10–100 micrometres, tens of thousands of times wider. Tiny air molecules scatter blue light far more than red (Rayleigh scattering), giving the sky its colour. But cloud droplets are so much larger than visible wavelengths that they scatter all colours of light almost equally (Mie scattering), so all the colours mix together and the cloud appears white.
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The whole story
How it works
When sunlight hits a particle much smaller than the wavelength of light (like an air molecule, ~0.3 nm), short blue wavelengths scatter far more strongly than long red ones — the Rayleigh 1/λ⁴ law means blue (~450 nm) scatters roughly 5–6 times more than red (~700 nm). This is Rayleigh scattering. Cloud droplets (~10–100 µm) are far larger than visible wavelengths, so scattering enters the Mie regime, which is nearly wavelength-independent — all colours scatter with almost equal strength. Equal amounts of all colours together look white.
What people get wrong
Clouds are NOT white because they reflect sunlight off their tops like a mirror. The white comes from light scattering inside the cloud itself — every droplet bounces light in all directions, and since all colours bounce equally, the result is white.
The catch
Thick clouds with many stacked layers of droplets scatter light so many times internally that very little escapes from the bottom — making the underside appear grey or dark.
Questions kids ask
Why are some clouds grey?
Grey clouds are very thick — so many layers of droplets bounce the light around that very little makes it through to the bottom, making the cloud look grey or dark.
Are clouds made of water vapour?
No — clouds are made of tiny liquid water droplets (or ice crystals at high altitude). Water vapour is invisible.
Why don't clouds look blue like the sky?
Air molecules (~0.3 nm) are far smaller than light waves so they prefer blue. Cloud droplets (~10-100 um) are huge compared to light waves so they bounce all colours equally, which blends to white.
Talk about it
- Next time you see a white cloud and a dark storm cloud, ask: what's different about them?
- Look at a glass of plain water — it's clear. But milk looks white. Why? (Hint: tiny fat droplets scattering light!)
For grown-ups
Rayleigh scattering (air molecules, d much smaller than wavelength) follows the 1/lambda^4 law, strongly favouring blue. Cloud droplets (d~10-100 um, far larger than visible wavelengths) scatter in the Mie regime, which is nearly wavelength-independent, producing white light.
Keep going
What else makes you wonder?
- Why do storm clouds look so dark and scary?
- How does a rainbow form if clouds just make white?
- Why do clouds turn orange and pink at sunset?