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Where does the blue sky end?
Answer 1:

First let me explain briefly why we see blue sky and then you will be able to answer your question yourself:
The sun produces white light, which is made up of light of all colors: red, orange, yellow, green, blue, indigo, violet. Light is a wave, and each of these colors corresponds to a different frequency, and therefore wavelength, of light.
When the white light from the sun shines through the earth's atmosphere, it collides with the gas molecules. These molecules scatter the light.
The shorter the wavelength of light, the more it is scattered by the atmosphere. Blue light is scattered ten times more than red light because it has a shorter wavelength.
When you look up in the sky, the scattered blue light is the light that you see.

So where does now the blue sky end? Since our atmosphere is responsible for the scattering of the light, the blue sky will end where the atmosphere ends. Our atmosphere reaches over 560 kilometers (348 miles) from the surface of the Earth.

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