All the images were processed using a custom Python script which will be made available in an upcoming post.
Rayleigh and Mie Scattering
Two primary atmospheric light scattering phenomenon are Rayleigh scattering, which gives the sky its blue color, and Mie scattering, which gives clouds their white color.
Mie is strongly forward scattering, meaning most of the light is scattered in the same direction as the incident light. It acts on particles roughly equal to or larger than the wavelength of light. As we will see, water molecules, dust, sand, and pollutants are all strong Mie scatterers.
Rayleigh is approximately equal forward scattering and back scattering, with reduced scattering perpendicular to the incident light. It acts most strongly on particles significantly smaller than the wavelength of light. Air molecules (nitrogen, oxygen, etc.) proportionally scatter short wavelength light—violet and blue—much more than medium and long wavelength light—green, yellow, orange, red. The shorter the wavelength, the stronger the scatter.
The delicate blue veil we see over the surface of Earth from space is Rayleigh scattering "bouncing" mostly blue light back to the observer, even though the direction of light is from behind.
Meteosat-12 Timelapse
12:00 UTC — Starting at solar noon, the disk of the Earth is most uniformly illuminated and crisp:
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| Meteosat-12 at solar noon. All images processed by the author. Credit: EUMETSAT |
We see comparatively little atmospheric Mie scattering, besides the clouds, because that light is traveling mostly away from the observer. At solar noon the Sun is directly behind the satellite. Rayleigh scattering, on the other hand, is quite strong in the direction opposite the incident light so we see the familiar blue veil.
10:00 UTC — Two hours earlier we see only subtle differences:
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| Credit: EUMETSAT |
08:00 UTC — Four hours earlier we see an overall darkening of the surface, with higher mountains standing out more distinctly from the surrounding lower elevations:
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| Credit: EUMETSAT |
07:00 UTC — Five hours earlier we begin to see dawn "advancing" on the left:
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| Credit: EUMETSAT |
06:00 UTC — Six hours before solar noon the terminator—the transition between day and night—is now clearly visible over the western Sahara:
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| Credit: EUMETSAT |
Warm sunglint over the Red Sea is becoming visible. Europe appears enveloped in a murky haze, making it almost indistinct from the oceans except for the higher elevations. But a person on the ground most likely would be seeing the increasingly bright pale blue morning sky, not some thick smog. The apparent difference is due to light/observer geometry and physics.
05:00 UTC — At seven hours Mie scattering is becoming the dominant phenomena, especially towards the limb:
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| Credit: EUMETSAT |
The lower albedo of much of Europe and the soupy veil of haze combine to practically obscure its features. The prominent Pontic and Taurus mountains ringing Turkey stand out clearly from the flatter, lower interior. Since we are looking more towards the direction of light now, Mie forward scattering is turning Iran and central Asia a pale light gray. Sunglint now extends to the Persian Gulf and beyond.
04:00 UTC — Finally, eight hours before noon:
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| Credit: EUMETSAT |
The Pontic, Taurus, Caucasus, Zagros, and Alborz mountain ranges in southwest Asia stand out in great relief due to the less dense atmosphere (thus less Mie forward scattering) and the simple fact that we are seeing mostly the shadow sides of the innumerable mountains and hills. In stark contrast, the lower relief of the terrain around the ranges cast fewer shadows and we observe mostly sunlit surfaces scattering all their suspended particulates towards the camera.
Final Thoughts
These images show the vast difference in overall surface reflectance with Sun phase angle. Would a human observer in space see it this way too? I suspect not quite, unless the display these images are viewed on is extremely bright. Think how bright the Moon looks even though its albedo is much lower than Earth's.
All the images were processed with the same brightness and contrast stretching parameters. To reproduce the physiological effect of pupils widening in lower light, for example, perhaps further stretching should be applied as the images get darker. It's a complicated, complex question.
Lastly, click on any of the images and use your mouse wheel to quickly scroll between them to see the motion of clouds and sand!
CREDIT
EUMETSAT
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
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