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The sky is blue, when looking from below. That's based on scattering, no problem with that: Why is the sky blue?

But it is also colorless transparent from above. No problem with that either, we look straight to the ground, there is only air in between.

So it has a color, and no color, at the same time. That is surprising!

Independent of color, that it is colored from only one side is strange. It is not opake blue from below, as we can see the sun.

That reminds of a half transparent mirror. It's not the same, as a mirror creates reflection, not scattering.
The blue color is created by scattering. Shouldn't scattering go to all directions?

How can the sky have two colors at the same time?

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  • $\begingroup$ It is not transparent from only one side, you can see stuff beyond atmosphere, e.g. (a part from the sun, obviously) the moon, some planets, comets etc $\endgroup$
    – fqq
    Commented Nov 18, 2019 at 11:38
  • $\begingroup$ And it's obviously (mostly) transparent at night $\endgroup$
    – fqq
    Commented Nov 18, 2019 at 11:38
  • $\begingroup$ @fqq For a strict definition of obvious, it's not so obvious. But let's assume the question implies it's night. $\endgroup$
    – Volker Siegel
    Commented Nov 18, 2019 at 11:49
  • $\begingroup$ Please do not post answers in the comments. If you know the answer to the question, please post it as a proper answer. Thanks! $\endgroup$
    – tpg2114
    Commented Nov 18, 2019 at 11:51

2 Answers 2

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From below you are looking up at the atmosphere with a black background beyond it.
From above the background is reflecting significant amounts of light and so you see the background.

This picture from a satellite shows the background visible but getting less so as the light passes through more and more atmosphere towards the rim.

enter image description here

Here is the effect at night (for most of Earth) with the rim showing the scattering of blue light from the atmosphere.

enter image description here

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  1. Blue (red and other) colors of the sky come from Rayleigh scattering effects: see https://en.wikipedia.org/wiki/Rayleigh_scattering . And no: its effects are not equal in all directions!
  2. White and grey colors of the clouds come from Mie scattering (https://en.wikipedia.org/wiki/Mie_scattering )
  3. Remember additive and subtractive color mixing difference. The different colors you see at the sky result from different effects.
  4. And last but not least: remember colors are just human perception of (mixed) frequencies.
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