Just before nightfall on Oct. 31st, Ed Kreminski of Westerville, Ohio, dashed to the store for emergency candy. Why? To feed the dinosaur:

"I spotted this dinosaur in the sky while I was getting ready for Trick-or-Treaters," he says. It was a great way to kick off Halloween.

The iridescent colors are caused by tiny water droplets in the cloud diffracting sunlight. And if you think a pastel dinosaur looks funny, think again. While the fossil record does not tell us the color of dinosaur skin, it does show that many dinosaurs are related to modern birds. (Some paleontologists say birds are dinosaurs.) This means dinosaurs may have been as colorful as a peacock, a Lilacbreasted Roller--or even an iridescent cloud. Keep looking up! 

SOURCE: Space Weather

The string theorist Jonathan Shock gave me a link to the Cloud Appreciation Society. That's where I got this photo of a nacreous cloud, taken by Hugo Patten:

These incredible iridescent clouds were observed around midday near Taunus, Germany, on March 6, 2007. Intense colors in altocumulus clouds rendered a sky that was breathtaking. The iridescent coloration results from diffraction of sunlight -- small water droplets composing these mid level clouds interfere with or deflect the Sun's rays. When observing optical phenomena, especially in the middle of the day, be sure to block out the Sun, otherwise your vision could be seriously damaged. See also the Earth Science Picture of the Day for December 1, 2006.

SOURCE: EPOD for June 30, 2007

An Iridescent Cloud Over Colorado
2007 November 25

Explanation: Why would a cloud appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. This iridescent cloud was photographed above Boulder, Colorado last week.

SOURCE: NASA APOD

Iridescent Clouds Over Aiguille de la Tsa
2003 October 14 

Explanation: Before the sun rose over the mountains, iridescent colors danced across the sky. The unexpected light show was caused by a batch of iridescent clouds, and captured on film in early September in Arolla, Wallis, Switzerland. The peak in the foreground of the above image is Aiguille de la Tsa. Iridescent clouds contain patches of water droplets of nearly identical size that can therefore diffract sunlight in a nearly uniform manner. Different colors will be deflected by different amounts and so come to the observer from slightly different directions. Iridescent clouds are best seen outside the glare of the direct Sun although they can occasionally be seen to encircle the Sun.

SOURCE:NASA APOD

Iridescent Clouds
2007 November 25

Explanation: Why would clouds appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. Pictured above, iridescent clouds were photographed over Norway two months ago.

SOURCE: NASA APOD

...
Iridescent Clouds over Thamserku
Image Credit & Copyright: Oleg Bartunov

Iridescent Clouds over Thamserku
2014 July 8 

Explanation: Why would a cloud appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. The above iridescent cloud was photographed in 2009 from the Himalayan Mountains in Nepal, behind the 6,600-meter peak named Thamserku.

SOURCE: NASA APOD

Here is a great capture of a similar iridescent plieus cloud on Youtube. Embedding is currently disabled but the video is fantastic: http://www.youtube.com/watch?v=8-H3xlKU1gA

Iridescence in Florida pileus clouds
July 2003

Imaged in July '03 by David LaPuma (site) using a Leica Televid 77 APO spotting scope.   The inset shows the area of the main image atop a towering cumulus cloud.  Pileus or "mushroom cap" clouds are formed above cumulus. During the day warm moist air in cumulus rises and the clouds tower upwards. Sometimes their vertical growth pushes up a layer of moist air above them. The air layer expands as it is forced up into lower pressure surroundings and adiabatically cools. The water vapour in it suddenly condenses forming a misty veil-like layer of droplets above the cumulus - pileus cloud. Any cloud formed suddenly has all its droplets of similar size - ideal for iridescence or a corona.

Source: Atoptics.UK

• Atmospheric Optics
• Atmospheric Optics: Iridescent
• Atmospheric Phenomena - Germany
• Cloud Appreciation Society
• Color and Light in Nature - Cambridge, UK
• EPOD for June 30, 2007
• EPOD for December 1, 2006
• Optical Phenomena - Cloud Iridescence
• Wikipedia - Cloud Iridescence