Sky (or celestial dome) is everything that lies a certain distance above the surface of Earth, including the atmosphere and the rest of outer space. In the field of astronomy, the sky is also called the celestial sphere. This is an imaginary dome where the sun, stars, planets, and the moon are seen to be traveling. The celestial sphere is divided into regions called constellations.
Usually, the term sky is used from the point of view of the Earth's surface; however, the exact meaning of the term can vary. For example, in some cases the sky is defined as only the denser portions of the atmosphere.
During daylight, the sky appears to be blue because air scatters blue sunlight more than it scatters red. At night, the sky appears to be a mostly dark surface or region scattered with stars. During the day, the Sun can be seen in the sky, unless obscured by clouds. In the night sky (and to some extent during the day) the moon, planets and stars are visible in the sky.
Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation can also be seen in the sky during storms. Birds, insects, aircraft, and kites are often considered to fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.
Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the forward and backward directions than it is in the lateral direction. Scattering is significant for light at all visible wavelengths, but it is stronger at the shorter (bluer) end of the visible spectrum; meaning that that the scattered light is more blue than its source, the sun.
The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue. Although violet, not blue, is the color with the shortest visible wavelength; the spectrum of light emission from the sun is not constant at all wavelengths, and additionally is partially absorbed by the high atmosphere, so there is less violet in the light. Human eyes are also less sensitive to violet than blue.
Scattering also occurs even more strongly in clouds. Individual water droplets exposed to white light will create a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed out white color. [ source ]