PORTLAND, Ore. (KOIN) — Portlanders may have noticed rainbow-colored clouds floating overhead Sunday afternoon. Viewers Hanna and Emma Buckingham provided these photos of the clouds, which they said were visible throughout their drive from Beaverton to Tualatin at 1 p.m.

KOIN 6 Meteorologist Kelley Bayern says that these rainbow clouds, also known as cloud iridescence, are a rare phenomenon that occur when water droplets and small ice crystals cause light to spread out into a spectrum of color.

“Cloud iridescence usually happens due to diffraction: When small water droplets or small ice crystals scatter the sun’s light,” Bayern said. “The cloud must be pretty thin and have lots of water droplets or ice crystals in it.”

The National Oceanic and Atmospheric Administration states that rainbow clouds are rare because multiple conditions must be present for the phenomenon to occur.

“The cloud must be thin and have lots of water droplets or ice crystals of about the same size,” NOAA says. “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.”