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Sovereignty

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Looks fake. I need more info.

I know of sun dogs and rainbows. This looks like an opal photoshopped over an unusually perfect flight of geese. ETA: that just happened to be flying by.
ETA2: I've seen iridescent clouds like those shown in this Wikipedia article, but they show/follow the ROYGBIV banding of the spectrum. I don't see that here.
ETA3: Cloud iridescence I've seen has always been limited in extent across the sky, like the Wikipedia examples. The twitter pic covers a large area.
 
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Innula Zenovka

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Sovereignty

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I was a bit suspicious, too, but it turns out that cloud iridescence is a thing, as Google will confirm.

EG

That's a nice a picture you found, but it shows two of the reasons why I'm dubious: small patch of cloud and distinct banding of colors. (Sorry for the ETA's. Bad habit.)

I'll go out on a physics limb. Another issue with the tweet is that it mentions diffraction. Ice and water droplets refract light. Different wave lengths bend different amounts going between air and ice/water. I'm only vaguely familiar with color and diffraction, but I think diffraction is a way more flexible mechanism for generating colors. For instance, blue jays are actually brown, but diffracted light makes them look blue. At least, that's what I read somewhere. If you turn over a blue jay's feather, the underside looks brown, their color without diffraction. Difference Between Diffraction and Refraction

As far as I know, ice crystals and water droplets are too large generally to be effective at diffraction. "Generally, diffraction effects are most pronounced when the dimensions of the obstacle nearly agrees with the wavelength of the wave." (ibid.)

I welcome corrections.
 

Innula Zenovka

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That's a nice a picture you found, but it shows two of the reasons why I'm dubious: small patch of cloud and distinct banding of colors. (Sorry for the ETA's. Bad habit.)

I'll go out on a physics limb. Another issue with the tweet is that it mentions diffraction. Ice and water droplets refract light. Different wave lengths bend different amounts going between air and ice/water. I'm only vaguely familiar with color and diffraction, but I think diffraction is a way more flexible mechanism for generating colors. For instance, blue jays are actually brown, but diffracted light makes them look blue. At least, that's what I read somewhere. If you turn over a blue jay's feather, the underside looks brown, their color without diffraction. Difference Between Diffraction and Refraction

As far as I know, ice crystals and water droplets are too large generally to be effective at diffraction. "Generally, diffraction effects are most pronounced when the dimensions of the obstacle nearly agrees with the wavelength of the wave." (ibid.)

I welcome corrections.
I'm certainly enjoying trying to find photos of iridescent clouds



Great slideshow here.

 
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Sovereignty

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Except when people do not understand what I've said and only respond with a disparaging emoji. Not sure how to engage them constructively except to mention their thoughtless use of an emoji with that emoji alongside, i.e., mention it in an ETA.

In other circumstances, I do add ETA's that I should have taken the time to think of before posting the first time, but I was just too eager to post something. I guess that's how a lot of thoughtless emoji's wind up attached to posts, too.
 

Sovereignty

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Being a life-long nerd, I have a book Clouds of the World, A Complete Color Encyclopedia (1972). It goes into the physics a bit and has a section on optical phenomena (like halos around your head in the grass), but its discussion of iridescence is in the wave cloud section.

Iridescence is seen in clouds whose droplet size is fairly uniform at a point. The colour seen depends on the angle from the sun and the droplet size. Close to the sun rings are seen concentric on the sun, but at greater angular distances the variations in droplet size become more important, and the colours therefore tend to follow the contours of a wave cloud in which, owing to the relatively weak upcurrent compared with a cumulus, only the more efficient nuclei grow droplets, there number density remaining fairly constant through the cloud.

Any wave cloud might, in principle, show iridescence but it is most clearly visible where the rest of the sky is dark. Mother of pearl clouds are at altitudes between 19 and 30 km. where they may be brightly illuminated by the sun after sunset at the ground, and after the sun as ceased to illuminate the troposphere where most of the blue colour of the sky originates. A relative shortage of nuclei may severely limit the number of cloud droplets and make their size rather more uniform than in troposphereic clouds.
If the original photo that started this discussion is real, then it most likely is a mother of pearl cloud since the sky is so dark. I still doubt it is real, but then I've never seen a mother of pearl cloud. The pictures in the book show large, flat bright clouds with little color and look nothing like the picture in question. The Wikipedia article "Polar Stratospheric Cloud", aka mother of pearl clouds, shows better pics which make the given picture more plausible since one example covers a large area of sky with billows in the wave cloud (smaller waves superimposed on a larger wave). Still ... the pattern of iridescence which looks so much like an opal suggests a quite complicated cloud, not just billows superimposed on a wave. Then there is the issue of timing a flight of geese, framed by tree branches, under a cloud whose colors are changing as the sun sets.
 

Sovereignty

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I take issue with the National Geographic headline "A Rare Look at an Iridescent Cloud".

It pretends seeing iridescence in pileus is rare. Pileus is a cloud similar internally to a wave cloud but formed as stable air is pushed up by some updraft like within a cumulus cloud, volcanic eruption, uh, nuclear explosion. Pileus is quite common. To see iridescence you just have to be standing in the right place, like with a rainbow.

People don't look at clouds as much as they should, and if you don't live where there are lots of summer-time thunderstorms, get your butt down here.
 

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Fascinating family history


It's a response to this thread

 

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Katheryne Helendale

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Good to see Rick Moranis again!

Yes, good to see him again. And that Mint Mobile deal looks pretty darned good, too. As soon as I'm in the financial position where I can afford to pre-pay my cell bill three months at a time, I might switch from Cricket.
 

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And that Mint Mobile deal looks pretty darned good, too. As soon as I'm in the financial position where I can afford to pre-pay my cell bill three months at a time, I might switch from Cricket.
It's not *that* pretty darn good. They're "unlimited" plan goes to $40 for 3 month prepay after the first 3. It stays at $30 "unlimited" if you prepay a full year. I do like that they slow data rates (but by how much?) rather than start charging extra if you go over on a plan during the month, so you never get an extra charge and you never fully lose access.