Rainbow Clouds and the Phenomenon of Cloud Iridescence

Context

Indonesia recently saw a rare weather event called a “rainbow cloud,” also known as cloud iridescence, which attracted public attention because of its bright, colourful patterns visible in the sky.

About Rainbow Cloud

1. It occurs when sunlight interacts with extremely small water droplets or tiny ice crystals suspended in clouds.
2. This optical effect is produced through diffraction, where light bends and spreads after encountering particles that are similar in size to the wavelength of visible light.
3. Due to this process, light is separated into multiple colours, creating visible pastel shades and rainbow-like patterns across cloud surfaces.
4. The effect becomes more distinct when cloud particles are very small and relatively uniform in size, allowing clearer separation of colours.

Conditions for Formation and Rarity

1. Cloud iridescence is relatively rare because several specific atmospheric conditions must occur together.
2. The clouds must be thin enough for sunlight to pass through, yet contain droplets or ice crystals that are nearly identical in size.
3. Even slight variation in particle size can weaken or completely prevent the colour formation.
4. The phenomenon also depends strongly on the position of the Sun and the observer’s viewing angle, which determines visibility.

Types of Clouds That Produce Iridescence

1. Iridescence is most commonly observed in newly forming clouds and semi-transparent clouds, where particle uniformity is higher.
2. It is seen in cloud types such as altocumulus, cirrocumulus, cirrus, and lenticular clouds.
3. Among these, lenticular clouds often show especially dramatic iridescence due to smooth shapes and stable airflow that support highly uniform droplet formation.
4. These clouds generally develop at high altitudes, where relatively stable atmospheric conditions allow consistent interaction between sunlight and particles.

Difference Between Rainbow Clouds and Real Rainbows

1. Although called “rainbow clouds,” they are not true rainbows.
2. Rainbows form when sunlight is refracted, reflected, and dispersed inside raindrops after rainfall.
3. In contrast, cloud iridescence is caused by diffraction and interference involving microscopic droplets or ice crystals.
4. Unlike the large arc shape of rainbows, iridescent clouds show soft, uneven colour patches, often closer to the Sun and less structured in appearance.