The sky is part of the surface from the surface to the outside. Part of the atmosphere and the space.
The sky refers to only the dense part of the lower atmosphere.
The sky looks blue for the distortion of light in daylight. And at night, the sky looks like a black carpet full of stars. If there is no cloud during the day, the sun can be seen in the sky. Sometimes the moon, planets and stars are seen in the daytime. Natural phenomena such as clouds, raindrops, polar bear, thunder, etc. are observed in the sky.
Due to the scattering of the light the sky looks blue. When the particles are illuminated, the particles spread on the light in different ways, which is called distortion of light. The less the light's wavelength, the greater the scattering of the light. The scattering of the wavelength of light scattering of light is intrusive. The blue light wavelength is the lowest, so the sky is more scattering in the sky and the sky looks blue.