Cloud Iridescence: Colorful Light via Small Water Droplets or Crystals

Summary: The phenomenon of cloud iridescence, colorful light via small water droplets or crystals, happened for Costa Rica's Independence Day, Sept. 15.

cloud iridescence over Costa Rica, September 2015: 101.5 TODAY RADIO @1015todayradio, via Twitter, Sep. 21, 2015

Cloud iridescence is an unusual phenomenon occasioned by the diffraction, or scattering, of light by clusters of small, uniformly sized ice crystals or water droplets in thin areas of clouds, especially at cloud edges. The cumulative effect of the encounter between light rays and each crystal or droplet is a rainbow of pastel or vivid colors. The effect is similar to colors displayed in oil films on water.

Iridescence occurs most frequently in higher altitude clouds, such as altocumulus, cirrocumulus, cirrus and lenticular clouds. Either the sun’s direct light or reflected light via the moon may trigger cloud iridescence. The sun’s blinding rays often interfere with daytime viewing of cloud iridescence.

Iridescence refers to the rainbow-like optical effect of light on a surface determined by angle of illumination and angle of observation. The word iridescence (Ancient Greek: word ἶρις, iris, “rainbow” + Latin suffix: -escent, “becoming, resembling”) derives from Iris, a goddess in Ancient Greek mythology who personified the rainbow.

Particle size and distribution as well as the cloud’s optical thickness determine the patchwork of colors presented by iridescent clouds. The combination of white light in high content with scattered colors accounts for the low-purity pastel nature of iridescent clouds.

Cloud iridescence occurred on Costa Rica’s Independence Day, Tuesday, Sept. 15, 2015. Reports of visibility of the iridescent cloud centered on San José, national capital of Costa Rica and provincial capital of San José province. Sightings also were reported over San José province’s western neighbor, Puntarenas province, in Parrita canton on Costa Rica’s central Pacific coast.

Angela Fritz, deputy weather editor for The Washington Post, describes the iridescence as taking place in a pileus, or “cap,” cloud, which tops a cumulus cloud in a flat, wispy shape during early stages of thunderstorm development. The sun’s position behind the developing, stormy cumulus cloud encourages easy observation of iridescence as light rays, scattered by the thin layer of crystals or droplets in the pileus cloud, present a spectacular, rainbow-colored palette.

Some observers of the iridescent cloud over Costa Rica on the Central American country's Independent Day celebration, Tuesday, Sep. 15, 2015, labeled the photos that they shared via Instagram with the hashtag #ElcieloExtraño (“strange sky”). The colorful display is only a natural phenomenon celebrating the passage of moonlight or sunlight through a thin-layered area of small, uniformly sized water droplets or ice crystals in a cloud.

The unexpected viewing during daytime is thanks to cooperation from the sun.

Megiston via YouTube Sep. 18, 2015: "Amazing 'end of times' lights form among clouds over Costa Rica"

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credit:

cloud iridescence over Costa Rica, September 2015: 101.5 TODAY RADIO @1015todayradio, via Twitter, Sep. 21, 2015, @ https://twitter.com/1015todayradio/status/646048778076581888

"Amazing 'end of times' lights form among clouds over Costa Rica," uploaded Sep. 18, 2015, by Megiston to YouTube, @ https://www.youtube.com/watch?v=4lEvNaMqZnQ

For further information:

101.5 TODAY RADIO @1015todayradio. "This looks pretty Awesome!!! ~ 'Cloud Iridescence' spotted in Costa Rica http://abcn.ws/1LrS43A." Twitter. Sep. 21, 2015.
Available @ https://twitter.com/1015todayradio/status/646048778076581888

Fritz, Angela. “End of days? Nope. Here’s the real science behind the ‘mysterious’ Costa Rica cloud.” The Washington Post > Capital Weather Gang. Sep. 18, 2015.
Available @ https://www.washingtonpost.com/blogs/capital-weather-gang/wp/2015/09/18/end-of-days-nope-heres-the-real-science-behind-the-mysterious-costa-rica-cloud/

Megiston. "Amazing 'end of times' lights form among clouds over Costa Rica." YouTube. Sep. 18, 2015.
Available @ https://www.youtube.com/watch?v=4lEvNaMqZnQ