1. Why Clouds Look Grey From the Ground but White From a Plane
On a day when rain seems likely, the clouds you see at an airport appear grey or dark. However, if you take a plane and break through the cloud layer to the top, the clouds look bright and white [1].
The cloud did not change its color in the middle of the flight. The difference is whether you are looking from below or above. When you look from above, you see sunlight that has bounced off the surface of the cloud without being weakened [1].
This fact—that color changes based on viewing position—is a key clue. First, we must ask: why are clouds white in the first place? Do they contain a special white substance? The answer is no. There is no special white material inside clouds. Let us look at what clouds are made of.
2. What Clouds Are Made Of: Water Droplets That Are Transparent
Clouds are made of many water droplets or ice crystals floating in the sky. A typical cloud droplet has a radius of about 0.01 mm. To become a raindrop with a radius of 2 mm, the volume must increase by 8 million times [2]. (Note that radius and diameter are different. The diameter of a cloud droplet is about 0.02 mm.)
These tiny particles gather in huge numbers, but their material is water. Water in a glass is transparent, yet clouds are white. This is the puzzle to solve.
NASA explains that condensed water, whether liquid or solid, absorbs very little visible light. Instead, it scatters light of all wavelengths equally, which makes it appear white [5]. This is why both clouds and snow look white.
The fact that light is barely absorbed is a crucial clue. If light is not absorbed, it does not disappear. It must go somewhere. So, how does light move when it hits these particles?
3. Why the Sky Looks Blue but Clouds Look White: Particle Size
The sky looks blue because tiny particles, like air molecules, scatter blue light strongly. On the other hand, large particles, like cloud water droplets, scatter light of all wavelengths with almost equal strength [3].
The Hong Kong Observatory explains that water droplets and ice crystals in clouds are about 10 micrometers or larger. This is much bigger than the wavelength of visible light. Because of this size, all wavelengths of visible light are scattered to a similar degree [4]. (The radius of 0.01 mm mentioned earlier is equal to 10 micrometers.)
If only blue light were scattered strongly, the sky would look blue. But if all colors are scattered equally, the mixture does not lean toward any one color, resulting in a white appearance.
However, just scattering all colors equally is not enough to create "white." How the scattered light gathers and reaches your eyes is the second key factor.
4. How Light Scattering Many Times Inside a Cloud Makes It White
Light inside a cloud does not scatter just once and stop. The Hong Kong Observatory explains that light scatters many times in all directions between numerous water droplets and ice crystals. All the scattered wavelengths combine, making the cloud look white [4].
To summarize, three things create the whiteness of clouds:
1. Water absorbs very little visible light.
2. The particles are large enough to scatter light without choosing specific colors.
3. Light scatters many times, mixing various colors before reaching your eyes.
There is no white substance anywhere. White is created by transparent water particles and the way light is scattered.
But if light keeps wandering inside the cloud, the light reaching the bottom should decrease. In the next section, we look at why cloud bottoms become dark.
5. Why the Bottom of a Cloud Looks Grey: The Same Scattering
The Met Office in the UK explains that the grey color of a cloud's base is the result of the same scattering process that makes clouds white [3]. As a cloud becomes thicker, it is harder for light to pass through, so less light reaches the bottom.
Rain clouds contain many cloud droplets and raindrops. As sunlight hits the water droplets, it keeps changing direction. This weakens the light traveling in the original direction. Therefore, the bottom of the cloud looks grey or dark [1]. The size of water droplets near the cloud base also affects the brightness [1].
This grey color is not due to pigment. The light has simply decreased, making it darker. The same mechanism that creates white is still at work.
The more times light wanders inside a cloud, the less light passes through. This is likely why the bottoms of thick rain clouds with many water droplets are especially dark.
So, what do clouds look like when seen from far away?
6. Why a Cloud Looks White From Far Away but Dark From Underneath
When you look at a fluffy cloud from a distance, it shines white and bright. This is because you are seeing light that has bounced back out of the cloud due to many water droplets [1].
When that cloud comes directly overhead and you are underneath it, the sunlight is blocked, so it looks dark [1].
This answers the initial question. At the airport, looking up at a rain cloud, you see light that has been weakened by scattering. When you are on a plane above the cloud, you see sunlight bouncing off the cloud surface without being weakened, so it shines white [1].
The whiteness of a cloud is not a fixed color of the cloud itself. It is a color determined by the relationship between the cloud, the light, and your position.
7. Try It Yourself: Compare the Bottom and Sides of Clouds
On the next cloudy day, look up at the sky. Compare the dark parts of the cloud bottom with the bright edges or sides. Do thicker parts of the cloud look darker?
Do not look directly at the sun; look only at the clouds. Try looking at the same sky at different times of the day. The difference in brightness might change.
If you have the chance to fly on a plane, look at the clouds from above through the window. Remember the difference in color compared to how they looked from the ground.