What's going on with this cloud?

[Catherine Grant]

Hi CAS members,
This is my first post! I’ve been cloud-spotting my last six months in Cambodia, where I’m living.
Could anyone help me with what’s going on with the cloud in this photo, which I took about 6pm on the south coast of Cambodia a couple of weeks ago? Are these just shadows, at the top? (They certainly weren’t clouds). Or crepuscular rays, going upwards?
Hard to see in the photo, but there were also fairly spectacular colours at certain points on the lining, shifting through the rainbow for a good 15 minutes or so. (a little visible in red on the left, in this photo).
Just curious to learn more!
Thanks!

[Gavin Pretor-Pinney]

Hi Catherine,
It’s a cloud shadow being cast down from the summit of a distant storm cloud. So yes, crepuscular rays. The shadow shows up as the sunlight is scattered by a thin layer of Cirrostratus cloud just below the summit. The shadow looks like it is going up because of the effect of perspective. It works like this:

We shared a similar one on our Instagram feed recently that had been submitted through the CloudSpotter app.

Cumulonimbus over Miami-Dade, US, by 'cwvessels'. Known as the King of Clouds, the Cumulonimbus storm cloud can reach 10 miles up into the atmosphere. The canopy at the top is know as 'incus', after the Latin for a blacksmith's anvil. Despite its appearance, the cloud's shadow is likely being cast *down* onto a thin layer of cloud beneath the summit. The effect of perspective as the light shines towards the camera is what makes these 'crepuscular rays' of sunlight appear fan outwards. #CloudSpotterApp #clouds #cumulonimbus

A photo posted by The Cloud Appreciation Society (@cloudappsoc) on Nov 15, 2015 at 1:08pm PST

The colours around the edge of the storm cloud are called iridescence. They’re caused by the sunlight being separated into its constituent wavelengths as it is diffracted around the droplets at the fringes of the cloud.

I hope you are enjoying Cambodia!

[Howard Brown]

Hallo, Catherine and welcome aboard. That was a great observation of yours, so thanks for bringing it to us. I certainly was curious and enjoyed learning from Gavin’s explanation.

On general things of this nature there is a favourite site for the CAS Forum:

http://www.atoptics.co.uk/

Go to the atoptics home page and click on Search at the lower right and you can find Crepuscular rays.

[Catherine Grant]

Thanks very much for your responses, Gavin and Hygge – makes sense and is very interesting!
I’ll look forward to exploring the atopics site, hygge – looks fun.
I have another cloud question but maybe should post it in another thread…
Cheers!

[Howard Brown]

Caveat emptor – Atoptics index

Catherine I should have said that Atoptics warns that its index is not perfect e.g. when Andrew Kirk posted an Alpenglow picture I wanted to go back to it later but had forgotten the word, beyond it started with A… I am sure the subject is discussed though.

[Author]

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