Explained: Strange Clouds over Costa Rica [Cumulonimbus Pileus & Iridescence]

Mick West

Administrator
Staff member
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A video being shared on social media shows an odd looking colorful cloud that looks a bit like a translucent disc hovering over a storm cloud.

It actually is a translucent disc hovering over a storm cloud. It's a type of cloud know as Pileus, or a cap cloud, which is formed above a cumulonumbus cloud (a storm cloud) by the strong updraft. Seeing a pileus cloud indicates that the storm cloud is growing rapidly.
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Source

The colors are a cloud phenomenon known as iridescence. This occurs in transparent clouds when the droplets in the clouds are all similar sizes (typically this occurs in very young clouds). The similar sized droplets scatter light of a particular color, and small variations in the size, combined with the angle from the sun, create irregular bands of color.
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Source:

A google search shows many similar phenomena:

https://www.google.com/search?es_sm=119&tbm=isch&q=cumulonimbus+cap+cloud+iridescence

For more details and examples, see:
http://www.atoptics.co.uk/fz503.htm
 
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M Bornong

Senior Member.
It's no wonder that people get freaked out by these natural formations when even the mainstream news presents it with headlines like this:


Mysterious Iridescent 'End of Times' Cloud Phenomenon Spotted in Costa Rica
Content from External Source
and poorly explained:

Though users on social media have various theories on what caused the light formation, including aliens or UFOs, experts said the stunning view was caused by a rare weather phenomenon called "cloud iridescence."
Content from External Source
http://abcnews.go.com/US/mysterious...ud-phenomenon-spotted-costa/story?id=33834206
 
Found this quote from an oil company engineer who was talking about oil spill rainbows. Since what we are looking at does not have that vibrant look of a water based rainbow it seems likely there is something else there, or a bad camera but the rest of the cloud looks normal....
"The layer of oil on the water spreads out so thin, its thickness reaches a small multiple of the wavelength of visible light. (A few micrometers, basically.) That means light entering the oil layer can only make a few waves before it hits the water interface. If the oil thickness is an exact multiple of the wavelength, it is able to reflect back from the oil/water interface constructively and you see that color. If the thickness is a multiple plus 1/2 wavelength, it will reflect destructively and not be seen. There's also a lot of in-between, but that's where the color comes from.

Example: Say the oil is uniformly 1.5 micrometers thick and you look at it perpendicular to the surface. This only convenient multiple of visible light wavelengths is 3 waves of 500nm blue-green . So what you'll see reflected back is a shimmery blueish green. Other colors will reflect before entering the oil or pass through the oil-water interface, and not affect the color you see.

Of course, you're not really looking at it straight-on, are you? You're looking at it from an angle -- and that angle varies somewhat on the near side of the puddle and the far side of the puddle. The difference in angle affects the distance the light has to travel through the oil in order to be reflected towards your eyes. So the near side of the puddle will have a different color than the far side. Usually the oil isn't uniformly thick, and you look at it from different angles, so you get a pretty good rainbow from all the different wavelengths of light that are able to reflect off different parts of the oil sheen."
https://www.quora.com/Gasoline-What-causes-the-rainbow-of-colors-to-appear-in-oil-spills-in-parking-lots
Content from External Source
 
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Trailblazer

Moderator
Staff member
The original photos colors seems to have a lot more common with this then the spectrum you see from a water mist or cloud.

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They do, because they are both iridescence , which is caused by the interference between light waves. As also seen in CDs, soap bubbles, etc. Rainbows are not caused by iridescence, they are caused by simple refraction.

Cloud iridescence is caused by a somewhat similar mechanism to the thin-film iridescence in oily puddles, which is why the colours form in similar irregular bands.

So yes, Phillip is right, the pictures of iridescent clouds have more in common with oily puddles than they do with rainbows. But both are caused by water droplets, not oil.
 
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deirdre

Senior Member.
So yes, Phillip is right, the pictures of iridescent clouds have more in common with oily puddles than they do with rainbows. But both are caused by water droplets, not oil
so you are saying when we see color in clouds, sometimes its a rainbow (refraction) and sometimes it's iridescence?
 

Trailblazer

Moderator
Staff member
so you are saying when we see color in clouds, sometimes its a rainbow (refraction) and sometimes it's iridescence?

Precisely. Very different mechanisms. It's not just rainbows that are caused by refraction, though. Solar halos, circumzenithal arcs and sun dogs are also refraction effects; whereas iridescent and the rarer nacreous clouds get their colours from interference patterns.
 

deirdre

Senior Member.
Precisely. Very different mechanisms. It's not just rainbows that are caused by refraction, though. Solar halos, circumzenithal arcs and sun dogs are also refraction effects; whereas iridescent and the rarer nacreous clouds get their colours from interference patterns.
but Micks second picture is iradescence. ?? Maybe the question would be more than, what makes the OP first picture "less vibrant" than the second pciture. ??
 

Trailblazer

Moderator
Staff member
but Micks second picture is iradescence. ?? Maybe the question would be more than, what makes the OP first picture "less vibrant" than the second pciture. ??
Could be the camera settings or could be a real effect. The more vibrant example appears to be a thinner cloud.

Iridescence is a more random, swirling patchwork of colors. The size, shape, and color purity of either display depend on the cloud particle size and distribution and on the optical thickness of the cloud. Generally the colors are of a pastel nature, with low purity, because of the high content of white light combined with the diffracted colors
Content from External Source
http://www.montana.edu/jshaw/documents/Coronas_ShawNeiman_AO2003.pdf


I'm kind of simplifying a bit. Cloud iridescence isn't quite the same as oil-on-water iridescence, because the latter is thin-film interference, but the priniciple is the same: different wavelengths of light destructively and constructively interfere with each other, making some colours stronger and some disappear.

Mick summarised the differences in this post, although I wouldn't say that thin-film interference is "entirely different" from the Mie scattering that causes cloud iridescence. Both result in interference fringes of colour in similar ways.
 
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deirdre

Senior Member.
Cloud iridescence isn't quite the same as oil-on-water iridescence, because the latter is thin-film interference, but the priniciple is the same
i agree.

and people describe things using different terms. For ex. i would never describe that second rainbow in my pic as "vibrant", but i'm fine if Jermakian describes it as such. My mother keeps describing primarily blue colors as green. so it's all good.

i do think his oil pics look like the second photo in the OP. but i'm good if he (or anyone else) sees it differently.
 
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