Clouds appear rainbow-colored, a phenomenon called cloud iridescence, primarily when the sun is near the horizon and the cloud's water droplets or ice crystals are uniform in size. This causes light to be diffracted in a unique way, creating the rainbow-like effect.
Understanding Cloud Iridescence
Cloud iridescence is an optical phenomenon caused by the diffraction of sunlight (or, rarely, moonlight) by small water droplets or ice crystals in clouds. It's different from a rainbow, which is caused by refraction and reflection of light in larger raindrops.
Here’s a breakdown of the key factors:
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Small, Uniform Droplets or Crystals: Iridescence is most often observed in altocumulus, cirrocumulus, and lenticular clouds. These clouds contain very small water droplets or ice crystals that are relatively uniform in size. The uniformity is crucial for the light to diffract coherently.
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Diffraction: When sunlight passes through these tiny particles, it bends or diffracts around them. The amount of bending depends on the wavelength of the light and the size of the droplets/crystals. Because the droplets are nearly the same size, different colors of light are bent at different angles, separating them.
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Sun's Position: The sun's position plays a crucial role. Cloud iridescence is generally best seen when the sun is near the horizon (sunrise or sunset) or hidden behind a cloud, since the bright sunlight can overwhelm the subtle colors. When the sun is just below the horizon, it lights the clouds from underneath, enhancing the effect.
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Iridescent Appearance: The result is a shimmering, rainbow-like effect with pastel shades of pink, green, and blue. The colors are often localized in patches or bands within the cloud, giving it an ethereal and beautiful appearance.
Distinguishing Cloud Iridescence from Rainbows and Coronas
It's important to differentiate cloud iridescence from other atmospheric optical phenomena:
| Feature | Cloud Iridescence | Rainbow | Cloud Corona |
|---|---|---|---|
| Cause | Diffraction by small, uniform water droplets/ice crystals | Refraction and reflection by larger raindrops | Diffraction by water droplets (usually larger than in iridescence) |
| Appearance | Shimmering, pastel colors in patches or bands | Distinct arcs of color (red, orange, yellow, green, blue, indigo, violet) | Series of colored rings around the sun or moon |
| Droplet Size | Very small and uniform | Larger and varied | Varies, generally larger than in iridescence |
| Sun/Moon Position | Typically near horizon or hidden | Behind the observer, opposite the sun | Directly behind the cloud |
In Summary
Clouds turn rainbow, displaying iridescence, when sunlight diffracts around very small, uniformly sized water droplets or ice crystals, especially when the sun is near the horizon. This creates a beautiful, shimmering display of colors.
