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I was flying over the Atlantic at night in near complete darkness. There was a white, bright full moon. It’s moonglade (the light reflected off the waters surface) was a orange-red color and not white. Is there any explanation for this?

I’ve attached an image. enter image description here

Magnetic Monopsone
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Hay
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1 Answers1

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You would probably be about 9 to 12 km high, above the bulk of the atmosphere. The blue part of the light from the moon is more diffused travelling through the atmosphere and back, leaving the red part to be reflected, whereas the white light directly from the moon to the plane passes through less of the atmosphere and was not diffused, hence did not become reddish.

The particular phenomenon that results in reddening of transmitted light is known as Rayleigh scattering. There is more information in this video.

Red sunlight in the evening is due to the blue part of the light having been diffused out.

If the sun is fairly high in the sky, you should see something similar with sunlight reflected in the ocean. From a Google image search, it appears that the sun's reflection appears yellowish.

Suzu Hirose
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  • One nit: the spectral radiance curve off the moon's surface (including input from both solar and terrestrial sources) is not flat. I agree that the bulk of the 'shift' to orange is due to atmospheric scattering of the reflected radiance signal. – Carl Witthoft Jul 13 '22 at 14:05
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    If 'the blue part is more diffused in the denser atmosphere' means that that is where the Rayleigh scattering occurs: that is incorrect. At sea level, the spacing between molecules is 3 nm, much less than optical wavelengths, and you can't scatter off a single atom. You need one wavelength separation to scatter from a single atom. Nick Lucid breaks it down here: https://www.youtube.com/watch?v=4HBuHX4-VU8&t=313s – JEB Jul 13 '22 at 16:37
  • You don't have to be way up high in a plane to see this. I'm pretty sure I've seen the same effect just standing on the beach at certain times of night... – Darrel Hoffman Jul 13 '22 at 18:53
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    The usual term is scattering, not diffusion. The particular phenomenon that results in reddening of transmitted light is known as Rayleigh scattering. – Ruslan Jul 13 '22 at 18:59
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    The light reflected from the ocean has taken a round trip through the atmosphere including one journey from the very top to the bottom, so the argument still holds. – Suzu Hirose Jul 13 '22 at 19:18
  • @Ruslan Thank you for the correction, I have amended the answer. – Suzu Hirose Jul 13 '22 at 22:32
  • @JEB small spacing between molecules doesn't preclude scattering. Rayleigh scattering happens in any non-periodic medium, regardless of its number density. (In periodic media like crystals and vacuum it cancels out.) For discussion on Rayleigh extinction see the reference here. And regarding scattering off a single atom, you totally can. The cross section will be tiny but still nonzero. – Ruslan Jul 13 '22 at 22:44
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    @SuzuHirose you seem to have missed my point. I was talking about your choice of the word "diffused". It should have been "scattered" instead. – Ruslan Jul 13 '22 at 22:47
  • @Ruslan your link to Rayleigh extinction explicitly shows a 1/density term, and you can't scatter off 1 atom if the nearest atom is 3 nm away because you can't resolve one atom. Rayleigh scattering is completely suppressed in the lower atmosphere, this is an observational fact: ships/islands on the horizon aren't red. Nor are distant mountains. – JEB Jul 13 '22 at 22:52
  • @JEB that's totally wrong. Ships/islands on the horizon are tens of kilometers away. That's negligible compared to the sunlight path of hundreds of kilometers from the observer to, say, Kármán line (when looking just over the horizon). If you try doing simulations in dedicated software, you'll find that the denser the atmosphere, the more Rayleigh extinction. See e.g. MYSTIC simulator from libRadtran, or read the theory in E.Bruneton's paper Precomputed Atmospheric Scattering. – Ruslan Jul 13 '22 at 23:00
  • @JEB as for the link, do note that the numerator contains square of refractivity (which is proportional to density for gases), so the total coefficient becomes directly proportional to density. – Ruslan Jul 13 '22 at 23:24
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    The blue halo you see is some of the blue missing from the red reflection, much like the blue sky during the day is the missing blue in someone else's sunset. – Peter - Reinstate Monica Jul 14 '22 at 08:32
  • @JEB this photo demonstrates how much sunlight reaches different altitudes at the same geographic location and constant solar elevation. (If the links gets broken, the name to search for is "Shuttle Plume Shadow Points to the Moon".) – Ruslan Jul 16 '22 at 22:49