Sky
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The sky is what we call the appearance of a hemisphere over our heads. On a clear day it appears blue. The deepness of the blue increases as we move from the horizon to the point above our head.
The sky is blue because of Raleigh scattering. Raleigh scattering defines the amount of scattering of light rays. Since all colors of the rainbow create a white light we should see a white sky, but blue light scatters much more than red. That is why the sky appears blue (on a cloudless day).
On a cloudy day, it appears to be grey.
