Have you ever seen a rainbow cloud? This rare phenomenon stuns people in China

People in China were stunned last month when they observed a rainbow cloud - a rare natural phenomenon called a pileus iridescent cloud.

According to the National Oceanic and Atmospheric Administration (NOAA), iridescent clouds happen because of diffraction - a phenomenon that occurs when small water droplets or small ice crystals scatter the sun's light.

The phenomenon is relatively rare, for which, the cloud must be thin and have lots of water droplets or ice crystals of about the same size. When that happens, the sun's rays encounter just a few droplets at a time. For this reason, semi-transparent clouds or clouds that are just forming are the ones most likely to have iridescence, according to the NOAA SciJinks website.

A picture of this pileus iridescent cloud was also featured on NASA's Astronomy Picture of the Day (APOD) website along with a brief explanation.

"Pictured, behind this darker cloud, is a pileus iridescent cloud, a group of water droplets that have a uniformly similar size and so together diffract different colors of sunlight by different amounts. Also captured were unusual cloud ripples above the pileus cloud. The formation of a rare pileus cloud capping a common cumulus cloud is an indication that the lower cloud is expanding upward and might well develop into a storm."

Run by the agency and Michigan Technological University, the APOD archive contains the largest collection of annotated astronomical images on the internet.