Clouds come in a variety of shapes, sizes, and colors. The colors we see in clouds give us clues about what’s going on inside them. But are the colors we see the “real” colors of clouds? Understanding cloud color and what creates it can tell us a lot about weather and climate.
What makes clouds look white?
Clouds appear white because they are made up of tiny water droplets or ice crystals that reflect and scatter light in all directions. The combination of all the visible wavelengths of light reflecting off the droplets makes clouds look white to our eyes.
When light from the sun enters a cloud, it encounters the water droplets within. These droplets are much larger than the wavelength of visible light – typically 10-50 micrometers across, compared to the 400-700 nanometer wavelengths of visible light. This means that the light cannot pass directly through the droplets. Instead, the light is reflected and refracted by the droplets, changing directions multiple times. This diffuse reflection in all directions makes the clouds look white.
The more droplets in a cloud, the more reflections occur. With more reflections, more of the light is scattered, making the cloud appear brighter white. Clouds with fewer or smaller droplets allow more light to pass straight through, making the cloud appear less white or more transparent.
How do clouds get color?
Although white is the most common color we see, clouds can take on many colors depending on the conditions inside them. Here are some of the main mechanisms that add color to clouds:
- Diffraction – Smaller water droplets or ice crystals can diffract light, separating it into colors.
- Reflection of ground objects – Clouds can reflect the color of surfaces below them.
- Sun angle – The angle of sunlight affects how much light is reflected and scattered.
- Cloud thickness – Thicker clouds reflect more light, appearing brighter white.
- Particle size – Larger droplets or crystals reflect light more effectively.
Diffraction
When cloud droplets get very tiny, comparable in size to the wavelengths of light, they can diffract light. Diffraction is the bending of light around small obstacles. Shorter wavelength blue light is diffracted more strongly than longer wavelength red light. This separates white light into colors and can make clouds appear blue or colorful around the edges.
Reflection
Clouds often take on the colors of the scenery or objects they are floating over. For example, clouds over a large forest will appear green or brown, while clouds over a city appear gray. The water droplets act like weak mirrors, reflecting some of the colors from below.
Sun angle
The angle at which sunlight hits cloud droplets affects how much light is reflected. When the sun is high in the sky, sunlight has a shorter path through the cloud. This means droplets reflect less light, making the cloud appear brighter white. During sunrises and sunsets, sunlight enters clouds at a shallow angle and has a longer path through them. This allows more light to be reflected and scattered, producing more intense oranges, reds, and yellows.
Cloud thickness
Thicker clouds with more layers have many opportunities to reflect and scatter light. This makes them appear brighter white when viewed from below. Thin, wispy clouds have fewer chances to interact with light, so more light passes through, making them appear dimmer or transparent.
Droplet/crystal size
Clouds made of larger water droplets or ice crystals reflect light more effectively. As a result, they appear whiter and brighter. Smaller droplets let more light pass through, reducing the white appearance.
Real color vs perceived color
The colors we see in clouds are what our eyes perceive looking up at them from the ground. But do these colors represent what the clouds would look like if you were inside them? In other words, what is the “real” color within a cloud?
The water droplets making up a cloud are colorless. However, their interactions with light cause the colors we see. So the perceived colors represent what is physically happening to the light inside the cloud. In that sense, the colors we see are the “real” colors of the clouds.
However, there are some processes that could make the colors appear different within a cloud:
- Light intensity – Less light penetrates into a cloud, muting the colors we might see from inside.
- Forward scattering – Light scatters in the direction it travels, increasing the brightness inside a cloud.
- Viewing angle – The side-view from the ground shows only reflected light, not the transmitted light through the cloud.
The reduced light level and side-viewing angle within a cloud mean the colors would likely be less intense than seen from the ground. Though the physical processes creating the colors would still be present within the cloud.
Typical cloud colors and what they mean
Different cloud types and conditions lend themselves to certain colors. Here are some typical cloud colorations and what they signify:
White
White or bright white clouds indicate larger droplets that effectively reflect sunlight. These are usually thick cumulus clouds.
Dark gray
Dark gray undersides suggest thick storm clouds with heavy rain potential. The darker color comes from less light penetration to the base of the cloud.
Bright white tops, dark gray base
Towering cumulonimbus clouds often take this familiar cauliflower shape. The white tops are high with large, icy crystals. Below is a darker, thicker layer with smaller droplets.
Orange, red, yellow
Warm colors are created when sunlight passes through smoke or dust particles, or when sunlight reflects at sunset/sunrise. They can indicate an approaching storm front.
Pink
Pink clouds occur when cloud droplets diffract reddish sunlight into colors. This usually happens around sunrise or sunset when red light dominates.
Blue
Blue colored patches are caused by diffraction preferentially scattering blue light. This typically occurs at the edges of thinner clouds.
Green
Clouds can turn green by diffracting blue and yellow light together. They may also take on a green tint when floating over forests.
Dark gray
Dark gray storm clouds bring the threat of heavy rain. The darkness results from reduced reflection off large raindrops within.
Interpreting colors to predict weather
Paying attention to cloud colors can help observers on the ground forecast approaching weather based on what’s happening in the clouds above them. Here’s how to interpret some common color combinations:
White puffy cumulus clouds
These fair weather clouds suggest clear, sunny skies. Their large droplets effectively reflect sunlight to appear bright white.
Increasing dark gray bases
As the cloud base darkens from gray to black, a storm is more likely. These darker colors mean thicker clouds and larger droplets.
Orange and red skies
Warm sunset colors indicate high clouds filled with ice crystals. Their large size diffracts red light. A storm may be on the horizon.
Morning red sky
If clouds appear red in the morning, it means they are thin enough for sunlight to shine through. Red light travels farther through cloud layers. A red sunrise reflects high pressure and stable air.
Dark spots on clouds
Localized dark patches can signal heavy rain as water droplets begin to coalesce. Watch for drizzle to follow under these spots.
Green clouds before storms
Ominous green clouds can form as water droplets diffract green and blue light together. Their dark green color is often an indicator of impending storms.
Conclusion
Cloud color is a complex phenomenon that depends on the size and distribution of water droplets and ice crystals within them. Diffraction, reflection, and scattering of light leads to the whites, grays, reds, oranges, and greens that we see. Paying attention to cloud color patterns can reveal information about current and approaching weather.
So next time you see colorful clouds in the sky, think about the optical effects producing those colors. The more we understand about how clouds interact with light, the better we can interpret the visual clues they provide about changes in our atmosphere.
Cloud color may be a product of our visual perception, but it is deeply connected to the physical processes within them. In meteorology and weather watching, cloud colors are as real and meaningful as the condensation and precipitation they signify.
| Cloud Color | Cause | Indication |
|---|---|---|
| White | Reflection and scattering by large droplets | Fair weather cumulus clouds |
| Dark gray | Absorption of light in thick, dense clouds | Storm clouds with rain likely |
| Orange, red, yellow | Diffraction around ice crystals or smoke particles at sunrise/sunset | Approaching storm front |
| Pink | Diffraction of reddish sunlight in high clouds | Fair weather cirrus clouds |
| Blue | Preferential diffraction and scattering of short wavelength blue light | Thin, wispy clouds |
| Green | Combined diffraction of blue and yellow light | Severe storms ahead |
Other cloud color questions
Here are answers to some other common questions about cloud color:
Why are clouds darker at the base?
Clouds typically contain larger droplets near the base which are more effective at absorbing and reflecting light, making the base appear darker. Smaller droplets at the top scatter more light, appearing whiter.
What causes rainbow clouds?
Iridescent rainbow clouds are caused by uniform-sized water droplets diffracting and refracting sunlight. The specific conditions required don’t happen often, making these clouds rare.
How does pollution change cloud color?
Pollution particles like dust, soot, and smoke can tint clouds yellow, orange, or red since they absorb and scatter blue light. More pollution makes clouds appear darker and murkier white.
Do cloud colors look different over land and ocean?
Cloud colors are generally similar over land and ocean. However, differences in average cloud thickness, droplet size, and viewing angles can lead to subtle variations in color.
Why do clouds look yellow before storms?
Several factors can contribute to yellowish clouds that seem to presage storms: sunlight scattering at high sun angles, increasing ice crystal size, and pollution particles from approaching weather fronts.
Can you ever see a cloud’s true color from the ground?
Not really. You’d have to be inside the cloud without viewing it from the side to see the true colors. Even then, the colors would appear muted compared to viewing from below due to less sunlight penetration.
Do red skies at night mean the next day will be clear?
Yes, red evening skies are a sign of high pressure and stable air which tend to bring sunny, clear weather the following day. This saying encapsulates weather folklore based on real optical phenomena.
Conclusion
Cloud color is a window into the interactions between light, water, and ice high above in the sky. The colors we see looking up are “real” in that they directly relate to physical processes occurring in the clouds overhead. By understanding what causes colors like white, gray, red and green, we can better predict weather from clues in the clouds.
So don’t just see clouds in black and white terms. Their colors add shading and texture to the story of our atmosphere. Reading the signs of condensation and light reveals much about upcoming weather. So next time you look up, appreciate the optical beauty of cloud colors and what they foretell.
