Cloud Iridescence

Today's update was going to recap my recent visits to Rocky Mountain National Park and Garden of the Gods, but yesterday I was treated to an exciting celestial display that I've never seen the likes of.

Yesterday afternoon I biked over to Mt. Sanitas, an outcrop of vertically tilted Dakota sandstone beds, for some trail running, bouldering, and photography practice (more on this later). While sitting at the bottom of a boulder, I glanced skyward to see a cloud passing near the sun, lit up with amazing shades of purple and cyan:

I didn't know it at the time, but this phenomenon is known as cloud iridescence (wiki article). When the sun's rays hit a rain or ice droplet in a cloud, the light diffracts, shifting the wavelength (and hence color) of the light that reaches the surface. In most clouds, light rays diffract many times before leaving the cloud which washes out any shift in color. In thin, wispy clouds (such as the cirrus clouds I observed), the light rays interact with only a small number of droplets so that the change color is preserved. The following close up shows the cloud and colors in more detail.

Part of the reason I went out to Sanitas was to play around with my new camera (a birthday present from my parents. Thanks parents!). But I wanted to make sure I photographed the cloud so I actually took the above photos with my iPhone - the new camera is the first I've owned with manual control over shutter speed and aperture and I'm not yet confident in my ability to quickly choose the right settings for a shot. After taking a bunch of photos with the iPhone, I took out my camera and managed to get a decent shot (after many failed attempts):

For those interested in photography, I had to go to push my camera's limits to capture the colors in the cloud, using an exposure and F-stop of 1/2000 s and F/8.0 (the min values for my camera, the Nikon S9700). If I let in any more light, the direct light from the sun washed out most of the color.

A bit later, a second cloud passed by. The interior of the cloud was too thick to iridesce, but the wispy edges show a splash of purple and cyan:

In addition to the amazing, glowing clouds, I took quite a few photos with the new camera. Here's a shot of the bouldering area known as Sanitas Proper:

I had a lot of trouble capturing both the blue skies and the rock in the foreground. Most of my photos either washed out the sky or underexposed the rock:

And finally, I'll leave you with the view south from the trail up Mt. Sanitas. You can see both Red Rocks (the crags on the left-most hill in the foreground) and the 1st Flatiron (the spike in the center of the peaks in the background):

More to come on the aforementioned trips to RMNP and Garden of the Gods...