Clouds appear multi-coloured, displaying a beautiful array of pastel or vibrant hues, primarily due to the diffraction of sunlight by exceptionally tiny ice crystals or water droplets within them. This optical phenomenon, often seen in high-altitude clouds, transforms ordinary clouds into a stunning natural spectacle.
The Science Behind Multi-Coloured Clouds
The key to understanding why clouds exhibit a rainbow-like effect lies in the size of the particles that make up the cloud.
The Role of Tiny Particles
Multi-coloured clouds are caused by particularly tiny ice crystals or water droplets in the air. When sunlight passes through these microscopic particles, it doesn't just pass straight through; instead, the light waves are diffracted – meaning they are bent and spread out.
This spreading separates white sunlight into its constituent colours, much like a prism. Because different colours of light have different wavelengths, they are diffracted at slightly different angles, resulting in the visible spectrum of colours we perceive in the cloud.
Diffraction vs. Halos
It's important to distinguish this from other atmospheric optical phenomena:
- Diffraction (Iridescence): Occurs with tiny (typically less than 10 microns in diameter) ice crystals or water droplets, causing light to spread out and create a rainbow effect directly within the cloud patches.
- Refraction (Halos, Rainbows): Larger ice crystals (for halos) or water droplets (for rainbows) interact with light differently. They primarily refract and reflect light, bending it at specific angles to create distinct arcs or rings around the sun or moon, rather than diffuse colours within the cloud itself.
| Phenomenon | Primary Cause | Particle Size | Appearance |
|---|---|---|---|
| Iridescence | Diffraction of light | Tiny (<10 microns) | Pastel or vibrant colours within cloud patches |
| Halos | Refraction and reflection of light | Larger (e.g., hexagonal) | Rings or arcs around the sun or moon |
| Rainbows | Refraction, reflection, and dispersion of light | Water droplets (rain/mist) | Arcs of spectrum colours opposite the sun |
For more insights into atmospheric optics, you can explore resources from meteorological organizations like the National Oceanic and Atmospheric Administration (NOAA).
Types of Multi-Coloured Clouds
The most common type of multi-coloured cloud is an iridescent cloud, also known as cloud iridescence.
Iridescent Clouds
These clouds typically appear as vibrant patches of pink, green, blue, and purple, often with soft, feathery edges. They are most commonly observed in:
- Altocumulus
- Cirrocumulus
- Lenticular clouds
- Polar stratospheric clouds (also known as nacreous clouds), which are exceptionally high and can display particularly brilliant iridescence due to the extremely small and uniform size of their ice crystals.
Conditions for Formation
For multi-coloured clouds to form and be visible, specific atmospheric conditions are usually required:
- Thin Clouds: The clouds must be relatively thin and contain a very uniform size of tiny water droplets or ice crystals.
- Sun's Position: The sun needs to be at a specific angle relative to the cloud and the observer, often hidden behind another cloud or just below the horizon. This allows the sunlight to just graze the edges of the tiny particles, maximizing the diffraction effect.
- High Altitude: Iridescence is often seen in high-altitude clouds where temperatures are low enough for small ice crystals to form, and atmospheric conditions can maintain their uniform size.
Observing these spectacular displays can be a breathtaking experience, reminding us of the intricate interactions between light and the Earth's atmosphere.
