The sky is blue, Sun is yellow, but the clouds are white! The Sun emits all of the colors in the visible spectrum. When you combine all of those colors, you get white. The Sun emits white light, and clouds contain tiny round water droplets. When that white sunlight passes through those tiny droplets, they are scattered in all directions. That’s why clouds are white.
The light from the Sun, which we all know is white, is a mixture of rainbow colors. We see different colors from that braid being split by reflecting part of that light absorption or some of that light or refraction, splitting apart that weave.
Why Are Clouds White?
Clouds appear white because of how they interact with sunlight. The white color of clouds is primarily due to the scattering of sunlight by water droplets or ice crystals. Here’s a simplified explanation of why clouds appear white:
Scattering of Light: When sunlight passes through the atmosphere and encounters cloud particles, it interacts with them in various ways. The particles are much larger than the wavelength of visible light, causing a scattering effect. This scattering occurs when light waves interact with the particles and change direction.
Multiple Scattering: Clouds contain numerous water droplets or ice crystals distributed throughout the cloud. As sunlight enters the cloud, it undergoes multiple scattering interactions with these particles. Each scattering event redirects the light in a different direction.
Scattering of Shorter Wavelengths: The scattering of sunlight by cloud particles is more efficient for shorter wavelengths, particularly the blue and violet portions of the visible light spectrum. These shorter wavelengths are scattered more readily, resulting in a predominance of scattered blue and violet light within the cloud.
Perception of White Color: While the scattering of shorter wavelengths contributes to the bluish tint of clouds, combining all scattered wavelengths results in a collective perception of white. The scattering of sunlight by cloud particles scatters and diffuses light in all directions, creating a mix of scattered light that appears white to our eyes.
Absorption and Transmittance: The cloud particles also absorb and transmit a small portion of sunlight. However, since the scattering of light dominates, the transmitted light is relatively weak, and the absorption is not significant enough to noticeably affect the overall color of the cloud.
Millions of tiny water droplets inside a cloud act like refraction prisms. They split the light apart into all of the colors of the rainbow. Water droplets do the same thing, sending these different colors into one another, effectively creating a mixture. What happens is that light gets blended back together like mixing paint colors until you get white.
Clouds are not consistent but do not have the same density. They’re not located at the same height, and they do not have the same color. Why is it that some clouds seem to be darker than others? Two main factors are significant in this phenomenon: sunlight and cloud density.
Besides the differences between the clouds, they have one thing in common. They all block sunlight, and sunlight travels through space in different wavelengths. Some of which are reflected in space. Once they hit the Earth’s atmosphere, some others are scattered. Once they enter the atmosphere, many wavelengths eventually reach the Earth’s surface.
Clouds consist of particles. The diameter is larger than the wavelength of a visible spectrum, reflecting in scattering light equally, making them wide. Water vapor floats into the air accumulating masses that are called clouds. Some of them are denser than others because they contain more water.
Visible light finds it challenging to pass through those clouds. It means that reflectivity is greater than invisibility. We may see them darker because we are standing beneath them. But they will appear white if you are on a plane traveling over those clouds.
As a result of the reflective ability, those clouds do not accumulate water vapors forever. They have a threshold, and water precipitates once this threshold is breached. That is why we have associated our clouds with rain. Depending on the density and atmospheric pressure, water does not always form clouds. They can create a haze sheet in the troposphere, the lower part of the atmosphere.
High levels of humidity are required for this phenomenon. Fog, smoke, and clouds are all formed from water droplets wandering around the atmosphere due to different atmospheric conditions. They all reflect the light of the visible spectrum equally, depending on their density. They sometimes appear white, sometimes darker in various shades of gray. The color is dependent on their opacity.
Why Are Clouds Grey And Darker Before The Rain?
Clouds are formed when air and water vapors near the ground warm up and rise. The water vapors condense as it increases, and the droplets join together to form clouds. The more condensation there is, the more droplets there are. When light from the Sun passes through these significant accumulations of water vapors, the droplets scatter the light in all directions.
The droplets are tiny and spread out enough to scatter the entire light spectrum, meaning they will appear white. The more water droplets gather, the clouds grow larger. Less light can penetrate through the cloud. What we see from the ground seems grey because less light is scattered to our eyes as the water droplets within the cloud get larger. This effect is enhanced, so clouds appear much darker before it rains.
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References:
Met Office, ed., “Difference Between Mist and Fog.”
World Meteorological Organization, ed., “Nimbostratus, International Cloud Atlas.”
Clouds Online. “Cloud Atlas.”
Koermer, Jim. “Plymouth State Meteorology Program Cloud Boutique.” Plymouth State University.
American Meteorological Society. “Glossary of Meteorology.”
Hatheway, Becca. “Cloud Types.” Windows to the Universe, US National Earth Science Teachers Association (NESTA).
