Timeline for Why is the sky *uniformly* blue?
Current License: CC BY-SA 4.0
13 events
| when toggle format | what | by | license | comment | |
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| Jun 26, 2020 at 14:04 | vote | accept | Vlad | ||
| Jun 24, 2020 at 16:49 | comment | added | Ruslan |
@Vlad besides, in my actual measurements of the sky radiance, I saw peaks at smaller wavelengths than violet. See the plot. Calculation of corresponding sRGB triplet yields #a8caff.
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| Jun 24, 2020 at 16:31 | comment | added | Ruslan | @Vlad That answer is misleading. The sky is not violet. Violet, as all the other colors, is a human-centric description of a set of spectral power distributions. If the sky were violet, we'd see it as violet—by definition of color. The main point of a good answer should be that one mustn't neglect the wavelengths away from the peak of spectral power density. If you calculate (by integration with color matching functions) the CIE 1931 color coordinates of the spectral distribution coming to you from the zenith, you'll get blue color, not violet. In fact, this is how my model predicts colors. | |
| Jun 24, 2020 at 14:19 | comment | added | Vlad | @Ruslan: slight offtopic: if I understood this question/answers, the sky is really violet but our eyes perceive the color as blue. However your model + computations predicts blue and not violet sky. Is this not a contradiction? | |
| Jun 23, 2020 at 14:19 | comment | added | Ruslan | @Vlad it's a real phenomenon caused by aerosols. This is because Mie scattering phase function has a strong forward peak with the width of about 5°. You can easily observe it in real clear sky if you hide your eyes from the Sun by some obstacle (e.g. a traffic sign) and look around the Sun's direction. You'll see something like this (a less exposed version here). Similar effect can be seen with the Moon (when it's bright in clear sky). | |
| Jun 23, 2020 at 13:59 | comment | added | Vlad | @Ruslan: Do I understand correctly that the white halo around the sun is not a consequence of less-than-perfect eye optics but follows from your model and computations? | |
| Jun 23, 2020 at 13:37 | comment | added | Vlad | @gerrit: Well, the color variations could be due to the non-perfect camera used for taking the photo. Well, I can look at the sky and witness the difference in color myself, but I still wonder where does all the red color disappear (so the gradient is blue-to-other-shadow-of-blue). | |
| Jun 23, 2020 at 12:51 | comment | added | gerrit | This could answer (+1) could be further improved by adding a photo of the sky from which the colour variations are immediately apparent. | |
| Jun 23, 2020 at 12:31 | comment | added | Vlad | Nice article! I assume it's worth mentioning in the answer itself, as the comments are often ignored by the majority of readers. | |
| Jun 23, 2020 at 12:15 | comment | added | Ruslan | @Vlad for a very good explanation of the calculations including single and multiple scattering please see E. Bruneton's paper Precomputed Atmospheric Scattering. My model is heavily based on this one. Even more information on calculations, as well as inclusion of ozone absorption, you can find in this update to the original example that came with the paper. | |
| Jun 23, 2020 at 12:09 | comment | added | Vlad | About sun in zenith: I used that assumption only for the geometrical setup explanation. Of course I'd prefer the computation in the general case for arbitrary position of Sun. | |
| Jun 23, 2020 at 12:00 | comment | added | Vlad | So you basically mean that the formula can be replaced with computation/modelling? If so, could you please shortly add some words about your mathematical model? | |
| Jun 23, 2020 at 6:53 | history | answered | Ruslan | CC BY-SA 4.0 |
