Rainbow Clouds Are Trending on Social Media, Here’s an Explanation from an IPB University Professor

The phenomenon of colorful clouds, known as “rainbow clouds” (iridescent clouds), is currently trending on social media. Photos and videos of the sky featuring beautiful color gradients have captured netizens’ attention, sparking curiosity about how this phenomenon occurs.

Addressing this, Sonni Setiawan, SSi, MSi, a lecturer in the Department of Geophysics and Meteorology at IPB University, explained that rainbow clouds are a natural atmospheric optical phenomenon.

“Iridescent clouds occur when sunlight is diffracted by water droplets or ice crystals measuring approximately 0,001–0,01 mm within the cloud. 

Since sunlight is polychromatic radiation, the diffracted light with different wavelengths will either reinforce or cancel each other out at certain angles, resulting in the colors red, green, blue, and violet that always appear at the edges of the clouds,” he said.

He noted that this phenomenon typically occurs in thin high-altitude clouds such as altocumulus and cirrocumulus clouds (thin clouds at altitudes of 5–10 km), lenticular clouds (lens shaped clouds over mountains), and stratospheric polaris clouds (pearl like clouds at the poles, with the most spectacular colors).

“The clouds must be thin. If they’re thick, the light is blocked and there’s no color effect. The thin cloud structure allows sunlight to penetrate and interact with the particles resulting in a shimmering spectrum of colors,” explained Sonni.

How Is It Different from a Rainbow?
Although it resembles a rainbow at first glance, Sonni explains that the process by which rainbow clouds form is different. While a rainbow is formed through the refraction of light, this phenomenon occurs due to the diffraction of light. Diffraction occurs when light passes through a slit or barrier whose size is comparable to the wavelength of light.

“In iridescent clouds, the ‘slit’ consists of water droplets or ice crystals with diameters of approximately 1–10 micrometers. This diameter is roughly equivalent to the wavelength of visible light, which ranges from 400 to 700 nm.

“If the droplets are too large, as in rain clouds, diffraction is not visible. If they are too small, as in clear air, the likelihood of diffraction is very low,” he said.

He explained that diffraction in these iridescent clouds follows the principle of single-slit diffraction, meaning that each droplet or ice crystal acts like a single slit that scatters light. The intensity of the diffracted light depends on the wavelength, the varying diffraction angles, and the diameter of the droplet or ice crystal particles.

“Since the wavelengths differ for each color, the diffraction angles and thus the positions of maximum intensity also differ,” he explained.

For example, blue light with a wavelength of 450 nm will be diffracted at a smaller angle than red light at a wavelength of 650 nm. As a result, the blue color appears closer to the light source, while the red color appears farther away.

Why Are the Colors Irregular?
In a rainbow, raindrops are uniform in size at ∼1 mm, and light undergoes refraction plus a single reflection. Thus, the pattern is regular, forming an arc.

Meanwhile, in an iridescent cloud, the droplets are very small and extremely numerous. Light undergoes double diffraction from many droplets. The interference patterns overlap, resulting in irregular patches of color at the cloud’s edges, rather than a perfect arc.

One of the most important points is that if the cloud droplets are not uniform in size, the diffraction patterns from each droplet will cancel each other out, resulting in a white, hazy color.

Iridescent clouds only form when the water droplets are uniform. This typically occurs in thin altocumulus clouds that have just formed and haven’t yet coalesced into larger droplets.

“The color spectrum does resemble a rainbow, but the process is different. In an iridescent cloud, light undergoes diffraction, not refraction like in a regular rainbow,” he explained.

Sonni noted that not all clouds can produce this phenomenon. Only particles of a specific size, comparable to the wavelength of sunlight, are capable of bending the light, thereby producing those colors.

“Not all cloud particles can diffract light. Only particles with a size matching the wavelength of sunlight can produce specific color effects,” he added.

He also noted that certain atmospheric conditions, including the prior presence of cumulonimbus clouds, can contribute to the formation of thin high altitude clouds that support this phenomenon.

“The rainbow cloud phenomenon can occur in various regions as long as atmospheric conditions are favorable. Therefore, the public can enjoy its appearance as part of nature’s beauty while also understanding the scientific processes behind it,” he said. (AS) (IAAS/LAN)