Sky

The sky commonly refers to everything that lies a certain distance above the surface of Earth, including the atmosphere and the rest of outer space. Usually, the term is used from the point of view of the Earth's surface. However, the exact meaning of the term can vary; in some cases, the sky is defined as only the denser portions of the atmosphere, for example. 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. On Earth, birds, insects, aircraft, and kites are often considered to fly in the sky. As a result of human activities, smog during the day and light radiance during the night are often seen above large cities (see also light pollution).
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.

1 Sky luminance and colors
2 Relationship with Weather
3 Importance in Astronomy
4 Significance in Mythology
5 Gallery

Sky luminance and colors

Clouds made orange by a sunset

When seen from high altitudes, the gradient in the sky's color is even more obvious: varies from pale to dark at elevations approaching the zenith

See also: Atmosphere of Earth

Except for light that comes directly from the sun, most of the light in the day sky is a result of scattering. Scattering is significant for light at all visible wavelengths, but it occurs more quickly at the longer (bluer) end of the visible spectrum. This means both that scattered light is more blue than the source, the sun, and that the source itself will appear slightly less blue. This is the reason that during sunsets and sunrises the sun may appear redder. However, the color of the sky is not the same everywhere. This is because light must travel through 38 times the atmosphere at the horizon than at the zenith, and so this Rayleigh scattering gives the sky a blue gradient — dark in the zenith, light near the horizon.Because red light scatters as well if there is enough air in between the source and the observer, shorter wavelengths of light will also scatter significantly, making parts of the sky change color during a sunset. As the amount of atmosphere nears infinity, the scattered light appears more and more white. The sun is not the only object that may appear less blue in atmosphere. Far away clouds or snowy mountaintops will seem yellow as well; that effect is not obvious on clear days, but very pronounced when clouds are covering the line of sight reducing the blue hue from scattered sunlight. This can be observed at the bottom part of the picture on top of the article. At higher altitudes, the sky trends to darker colors, since scattering is reduced due to lower air density; an extreme example is the moon, where there is no atmosphere and no scattering, making the sky on the moon black even when the sun is visible.
The scattering due to molecule sized particles (as in air) is almost random. The scattering in a 90 degree angle is still half of the scattering that reflects or goes forward. This causes the blue sky to be almost evenly colored and thin clouds to form a white area around the sun, because the big particles the clouds are made of are scattering preferentially only at low angles. The color of the clouds is also due to scattering and a cloud at a small distance has the white color because all the light from these clouds is scattered multiple times in the mass of particles and no wavelength effects will be observed.
The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) and black at night. Scattering effects also partially polarize light from the sky, most pronounced at an angle 90° from the sun.
Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. Recent developments relate to “all sky models” for modelling sky luminance under weather conditions ranging from clear sky to overcast.