Look at the cloud iridescence…oh, it’s gone
Early Thursday afternoon (October 15) I came out of the lane on the east side of South Granville Street at 10th Avenue and looked up at the southwest quadrant of the deep blue sky.
There was the glaring sun and a few wispy clouds, one of which was touched with the most vivid rainbow colours.
Scrambling, I tried to take a photo but the camera reported an SD card error. That sent me looking to see if I’d left the card in the laptop — I hadn’t.
Time marches, weather you are ready or not
The SD card was fine when I restarted the camera and I managed to get a photo of the fast-fading phenomenon at exactly 1:58 p.m.
I kept snapping but only ended up with one more decent shot before there was nothing left to photograph.
By 2 p.m., the colours had disappeared, along with much of the affected bit of cloud.
What I saw appears to match Wikipedia’s entry on cloud iridescence, a relatively rare optical phenomenon caused by small droplets of water or crystals of ice diffracting or scattering light.
As per the description, Thursday’s iridescent “rainbow cloud” occurred close to the glaring sun and was easier to see after I blocked the sun from my line of sight behind a utility pole.
Rainbows, by the way, are described as being caused by reflection and refraction, rather than diffraction.
Why are there pretty colours in the sky?
All three phenomenon: reflection, refraction and diffraction, result from light encountering and interacting with boundary conditions. Individually and in combination the three, along with interference, account for most of the colourful displays that we see in the sky, such as sunsets, rainbows, coronas and iridescence.
Reflection is when light rays bounce off of an object and it can play a role in creating rainbow colours when some of the colour wavelengths of a light ray are absorbed, leaving only specific colours to reflect back at the viewer.
Refraction is when the fish in the water are not quite where you see them, because the light rays reflecting off the fish are forced to all change direction (refract) as they cross the boundary of dense water into less dense air.
Refraction can create distinct colours because the exact angle of refraction through a medium of given density can be different for each of the colour wavelengths that make up white light (see refracting prisms).
Diffraction is like what Angelina Jolie and all the other assassins in the 2008 film Wanted were effecting when they willed their bullets to curve around obstacles. It’s a function of the wave-like nature of light rays, with longer wavelengths having the ability, because of their length and amplitude, to actually bend around obstacles. Click the images to enlarge them.