Why do rain clouds look black?
― There's no black paint involved anywhere
White clouds and black clouds are both made of the same thing: tiny water droplets. The difference in colour isn't about the type of droplet. It comes down to just two things — how thick the cloud is, and how far the sunlight has to travel through it. Between those two, a cloud's colour can shift from white to grey to black.
On a clear day, a cotton-wool cloud floats in the sky. It's almost dazzlingly white.
Then, in the evening, another cloud drifts into the same sky. Its underside is grey, and near the middle it looks almost black. Somehow you just know: "that one's going to rain."
But it's odd, isn't it? No one mixed black paint into the cloud. Both clouds are really just collections of the same water droplets.
Only two things decide a cloud's colour
Cloud droplets scatter light in every direction. Most of the scattered light heads back upward. So the thicker the cloud, the less light makes it all the way down to the ground.
We never look at a cloud in isolation. We see it alongside a dazzling sky, or a cloud top glowing white. Next to that, even a mildly dim grey reads as "black."
In other words, a cloud's colour isn't a property of the cloud itself — it's decided by the path the light took to reach your eye. Let's take it step by step.
Cloud droplets don't play favourites with colour
The sky is blue because air molecules are so tiny that they scatter blue light far more than other colours. Cloud droplets, though, are much bigger — actual drops of water, larger than the wavelength of visible light.
At that size, the differences between colours almost disappear. Red, green and blue are all scattered about equally. Mix all the colours together and you get white. That's why clouds are white, why fog is white, and why shaved ice is white.
So the same thing must be happening inside a black cloud too. What differs is how many times the light gets bounced around.
Inside a thick cloud, light loses its way
Light that enters a cloud can't travel in a straight line. Every time it hits a droplet it changes direction, then hits another droplet, and another. After enough back-and-forth, most of the light ends up heading back the way it came — upward.
In a thin cloud, light only needs to bounce a few times before it escapes downward, so from underneath it still looks bright white. But as a cloud gets thicker, less and less light manages to reach the bottom. Look at Figure 1. In the thin cloud on the left, light passes all the way through; in the thick cloud on the right, most of the arrows bend back upward.
Look at that same cloud from above, out of an aeroplane window, and it looks like a dazzlingly white carpet. Same cloud — white from above, black from below. That alone proves clouds aren't absorbing light; they're bouncing it away.
The other reason lies in our own eyes
The human eye isn't a machine that measures absolute brightness. It judges "bright" or "dark" by comparison with its surroundings. The very same grey looks dark next to something white, and bright next to something dark.
The base of a cloud usually sits right next to a bright sky or a glowing white cloud top in your field of view. So a patch that's really just grey ends up registering as unmistakably "black." Photograph it and measure the actual brightness, and it's often not nearly as dark as it looked.
When water droplets in a cloud grow into raindrops, the same amount of water ends up split among far fewer droplets. That shrinks the total surface area blocking the light, so light actually passes through more easily. Even so, a cloud about to rain looks dark because by the time the droplets have grown, the cloud itself has become very thick. The thickness effect wins out by a wide margin.
As the sun sets, its light has to cross a much longer stretch of air to reach us. Blue light gets scattered away along the route, leaving mostly reddish light to strike the underside of the cloud. Change the colour of the light hitting the cloud, and the cloud's apparent colour changes too.
Summary
A cloud is a collection of white water droplets that scatter light without favouring any colour. So there's no black ingredient in the cloud itself. As a cloud thickens, light bounces back upward again and again inside it, and only a sliver makes it to the bottom. Add in the eye's habit of judging brightness by comparison, and the cloud's base reads as black.
A black cloud isn't a cloud that changed colour.
It's a thick white cloud that light can no longer get through.
For more on how sky colours work, see Why is the sky blue, but sunsets red? For how cloud droplets form and why they don't fall, see Clouds are made of water — so why don't they fall? The same "scatter and turn white" mechanism is covered in Ice is transparent, so why does snow look white? And for how cloud heights swap around with the seasons, see Why does the autumn sky look higher than in summer?
- Fill a glass with water and stir in a single drop of milk. You'll get cloudy white water — a miniature cloud.
- Shine a light straight down on the glass and look at it from the side. The top glows brightly, while it gets darker toward the bottom. That's the thickness effect at work.
- Add a few more drops of milk, and the gap between top and bottom grows even bigger. Same white cloudiness, yet the bottom sinks into grey.
You can check this in the sky too. As a thick cloud passes overhead, look for its thin edges — those are the only parts glowing bright.
Want to go deeper? ― terms, formulas, and where this fits in textbooksLabelled from middle-school science through university-level courses, so you know what level each part is
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics (Light and Waves)"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level atmospheric radiation
- ResearchNot yet settled even at university level — an active research question
MSTerminology: this phenomenon has a name
- Mie scattering: the way light scatters when the particle size is comparable to or larger than the wavelength of light. There's almost no colour dependence, so it looks whitish.
- Multiple scattering: light doesn't just scatter off one droplet and stop — it scatters off the next droplet, then the next, again and again. This is what happens inside a thick cloud.
- Optical thickness: a measure of how much a substance blocks light as it passes through. It's determined by the cloud's physical thickness and how many droplets it contains.
- Reflectance: the fraction of incoming light that gets bounced back. On the sunlit tops of well-developed clouds, this value gets very high.
MSHSCheck it with a formula: what happens when thickness doubles?
Let's use a simple model: every fixed distance the light travels through the cloud, the amount heading downward is cut in half. Here, we'll say it halves every 100 metres. This isn't the literal physical property of real clouds — it's just a way to get a feel for how strongly thickness matters.
| In symbols | I ＝ I0 × ( 1/2 ) ^ ( L ÷ h ) |
| In words | Light reaching the bottom ＝ light at the cloud top × one-half repeated ("cloud thickness ÷ halving distance") times |
| Where this comes from | The idea that light loses the same fraction over each equal stretch of distance. It's the "Beer–Lambert law" for how light weakens, rewritten in terms of a halving distance. |
| Symbol | Meaning and unit |
| I0 | Strength of light hitting the cloud top (take this as 1) |
| I | Strength of light reaching the cloud base (same unit as I0) |
| L | Cloud thickness (metres) |
| h | Halving distance for light (metres) |
| Value used for symbol: a rough thickness for a somewhat thick cloud | 500 metres |
| A rough thickness for a well-developed cloud | 1000 metres |
| Halving distance for light (assumed for this model). Unit: metres | 100 metres |
| For a 500-metre cloud, number of halvings | 500 ÷ 100 ＝ 5 |
| Two halvings: what fraction of the original | 2 × 2 ＝ 4 |
| Three halvings | 4 × 2 ＝ 8 |
| Four halvings | 8 × 2 ＝ 16 |
| Five halvings | 16 × 2 ＝ 32 |
| Fraction of light reaching the bottom | 1 ÷ 32 ≒ 0.03 |
| As a percentage | 0.03 × 100 ＝ 3 |
So roughly 3 percent gets through to the bottom. Run the same calculation for a 1000-metre cloud, and the number of halvings becomes 10, so the light reaching the bottom is roughly 1 in 1000 — about 0.1 percent. Just doubling the thickness makes the darkness drop by an order of magnitude. That steep fall-off is why a cloud's base looks black.
HSHS+The relationship between droplet size and scattering
HSHow light scatters depends on the ratio between the particle size and the wavelength of light. When particles are much smaller than the wavelength, shorter wavelengths scatter more strongly, which is why the sky is blue. When particles are comparable to or larger than the wavelength, all colours scatter about equally. Cloud droplets fall into this second category.
HS+Cloud droplets are typically around 0.01 millimetres across. The wavelength of visible light is only about a hundredth of that. At that ratio, colour differences essentially vanish. Raindrops, meanwhile, grow to around 1 millimetre across. For the same total amount of water, larger droplets mean a smaller total surface area, so their ability to block light drops.
UnivSeen through radiative transfer
In atmospheric radiation science, light passing through clouds is handled with the radiative transfer equation. Inside a cloud, scattering vastly outweighs absorption, and for visible light, absorption is essentially negligible. In other words, a cloud isn't extinguishing light — it's just redirecting it. The reflectance of the sunlit top of a well-developed cloud becomes very high, and that's exactly how much less reaches the underside. This behaviour is also treated as an important factor in estimating the Earth's overall heat balance. The simplified case that ignores scattering and considers only absorption is the "Beer–Lambert law" used in the calculation above. For clouds, you add the effect of scattering to that and solve the radiative transfer equation instead, and cloud thickness gets expressed as a quantity called "optical thickness."
📖 For the derivation and further reading: Beer–Lambert law (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How to handle unevenness inside a cloud Real clouds aren't uniform — dense and thin patches are interwoven. How much this "patchiness" affects the total amount of light a cloud reflects is still considered a difficult thing to estimate.
- The effect of fine dust particles When the air contains a lot of fine dust, water droplets form smaller and more numerous. How this changes a cloud's brightness and lifespan is considered one of the major uncertainties in climate prediction.
- How precisely satellites can read cloud thickness Methods are used to estimate a cloud's thickness and water content from its brightness as seen from above. But for thin or patchy clouds, a non-negligible margin of error is said to remain in those estimates.
In other words, this article too describes things "as best understood so far." A familiar question like the colour of clouds connects straight through to the frontier of climate research.
Where this fits in textbooks (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science: reflection of light / weather observation | How cloud droplets bounce light back |
| HS | Physics Basics: light and waves | The relationship between droplet size and scattering |
| HS+ | Advanced physics/earth science / sidebar | Droplet size and total surface area |
| Univ | Atmospheric radiation / meteorology | Radiative transfer and cloud reflectance |
| Research | Climate science / satellite observation | Cloud patchiness and fine dust effects |
| ― | Everyday connection | Reading the sky to judge if it will rain |
- Japan Meteorological Agency website, "Types of Clouds" and related explanatory pages (気象庁ホームページ「雲の種類」).
- Yoshimitsu Ogura, General Meteorology (小倉義光『一般気象学』), University of Tokyo Press — chapters on cloud droplet size, growth, and optical properties.
- Kiyotaka Shibata, The Meteorology of Light (柴田清孝『光の気象学』), Asakura Publishing — fundamentals of atmospheric scattering and radiative transfer.
- Grant W. Petty, A First Course in Atmospheric Radiation, Sundog Publishing (an introduction to scattering and radiative transfer).
※This article is a general-audience science explainer. The figures given are approximations meant to build intuition, not precise measurements. When judging weather from the look of the sky, always follow the latest information and instructions from the Japan Meteorological Agency or your local authorities.