Atmospheric Optics – Iridescent Clouds

Iridescent clouds, also known as irisation, occur when thin parts of clouds display soft pastel colors. This phenomenon is caused by diffraction and interference of sunlight or moonlight by tiny, uniformly sized water droplets or ice crystals. The effect is most commonly observed near the edges of thin clouds, especially when they are newly formed.

Mechanism
When light encounters droplets or ice crystals of nearly uniform size, diffraction bends different wavelengths by slightly different amounts. These diffracted waves interfere with each other, producing visible colors. Smaller droplets produce a larger angular spread of colors, while larger droplets result in smaller, tighter patterns.

Favourable Conditions

• Thin, optically transparent clouds with low optical depth.

• Droplets or ice crystals with narrow size distribution.

• Locations at cloud tops or edges, often in altocumulus, cirrocumulus, lenticular, or pileus clouds.

Related Optical Phenomena

• Coronae: Concentric colored rings around the Sun or Moon caused by diffraction from tiny, uniformly sized droplets or ice crystals. The radius of the corona is inversely proportional to particle size.

• Glory: Concentric colored rings opposite the Sun, surrounding the observer’s or an aircraft’s shadow, caused by backward diffraction from water droplets.

• Halos and Sundogs (Parhelia): Formed by refraction through hexagonal ice crystals at fixed angles (commonly 22°), distinct from diffraction phenomena.

Key Distinctions

• Iridescence and coronae are diffraction-based phenomena, while halos and sundogs are refraction-based.

• Iridescence is usually irregular and patchy, while coronae appear as concentric rings.

• Glories occur opposite the Sun; iridescence and coronae occur near it.

UPSC-Relevant Facts

• Smaller droplets → larger corona radius.

• Uniform droplet size → sharper, more vivid colors.

• Thick clouds or wide droplet size range lead to muted or absent colors.

• Common in mid- and high-level clouds during early stages of development.

Exam Tip
Identify the mechanism (diffraction vs. refraction) and location in the sky (near Sun/Moon vs. opposite) to distinguish between similar atmospheric optical phenomena.