Clouds appear white during the day because they reflect all the colors of sunlight. In the evening, at sunset, clouds can appear red because the sunlight passing through a greater thickness of atmosphere is filtered, allowing the longer, redder wavelengths to pass through.
When sunlight passes through our atmosphere, it encounters tiny particles and air molecules along its path. These tiny obstacles scatter the light in all directions: this is called scattering. Sunlight appears white, but it is actually made up of all the colors of the light spectrum, each color having a specific wavelength. Colors with shorter wavelengths, like blue and violet, are scattered much more easily than colors with longer wavelengths like red. As a result, during the day, our eyes mainly pick up this blue light scattered everywhere, which is why the sky appears blue to us. In the evening, the story changes a bit, but that's another matter!
When the sun is high in the sky, its rays pass through a thin layer of atmosphere. The result? White light is less filtered, and the clouds appear very white. At sunrise or sunset, it's different. Here, the sun's rays arrive at a shallower angle. They pass through a thicker layer of atmosphere, which filters out more blue colors and mainly allows red and orange colors to pass through. That's why your nice white clouds then become wonders of red, pink, and orange hues.
Aerosols and other floating particles in the air play a significant role in the color of clouds. These small particles can be dust, pollen, pollution, or even microscopic water droplets. They disperse light rays differently depending on their size and characteristics: the more particles there are, the more intense and vivid the colors of the sky become at sunrise and sunset. When we observe brightly red or orange clouds in the evening, it is often because there are a lot of aerosols in the atmosphere. Conversely, the fewer particles there are, the paler or less saturated the colors appear.
Rayleigh scattering occurs when air molecules, tiny compared to the wavelength of visible light, primarily deflect shorter wavelengths, such as blue or violet. That's why we see a bright blue sky during the day. In the evening, it gets interesting: as light passes through a greater thickness of atmosphere, this scattered blue gradually disappears from our sight, allowing longer wavelengths like red and orange to reach our eyes directly.
But when light hits larger particles, like dust, droplets, or aerosols, we refer to Mie scattering. This type of scattering deflects all wavelengths similarly, which is why clouds, made mainly of large water droplets, appear predominantly white during the day. At sunset, these same clouds take on reddish or orange hues because only those warm-colored rays can reach them after passing through a thick layer of atmosphere. That's the whole spectacle.
The atmosphere sometimes acts as a filter that absorbs certain colors more than others. For example, gases such as ozone or water vapor absorb specific wavelengths of sunlight. This selective absorption significantly alters the color we see: some colors partially disappear, while others reach us perfectly. During the day, the effect is rarely very pronounced, as the sun is high in the sky and its light passes through less atmosphere before reaching us. However, at sunrise or sunset, the light travels through a thicker layer of atmosphere: more absorption occurs, some colors stand out better than others, resulting in those beautiful red or orange hues.
The practically nonexistent lunar atmosphere explains why the lunar sky consistently appears black, even in broad daylight, and why astronauts observe the lunar surface under very harsh lighting with no atmospheric diffusion.
The blue color of the sky that we see during the day is due to Rayleigh scattering, a phenomenon that disperses shorter wavelengths (blue) more than longer wavelengths (red or orange).
Contrary to popular belief, clouds appear white not because they are made of pure water vapor, but due to the numerous water droplets or ice crystals they contain, which scatter sunlight equally in all directions.
On Mars, sunsets appear bluish due to the different composition of the Martian atmosphere, which is primarily made up of carbon dioxide (CO₂) and diffuses light differently compared to Earth's atmosphere.
Near cities, the high concentration of air pollution particles and aerosols alters the interaction of light with clouds. These additional particles further scatter red and orange wavelengths, producing unusually vivid and intense colors during urban sunrises and sunsets.
The sky appears blue due to selective scattering known as Rayleigh scattering: air molecules preferentially scatter shorter blue wavelengths. In contrast, the water droplets in clouds scatter all wavelengths uniformly; the combination of all these colors is perceived as white by our eyes.
After a volcanic eruption, the atmosphere becomes filled with large amounts of volcanic aerosols and fine particles. These particles scatter and absorb sunlight differently, intensifying the red, orange, and pink hues of sunsets for several months or even years after the eruption.
The gray or black color of certain clouds comes from their thickness and water density. A dense, thick cloud blocks more light and does not reflect it as much as a thin white cloud. Therefore, the base of the cloud receives less direct light, giving it a dark or gray appearance when viewed from the ground.
No, seen from space, clouds generally appear white because they primarily reflect the entire spectrum of incoming sunlight. However, depending on the angle of observation, their thickness, and the presence of other surrounding atmospheric gases, their appearance and brightness can vary slightly.
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