Sky

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For other uses, see Sky (disambiguation).

A typical daytime sky.

Turbulent skies

The sky as seen from an airplane.

Stormy skies

Daytime sky and trees

Clouds made orange by a sunset

Sky at 30,000 feet (9 Km) altitude.

The sky is the part of the atmosphere or of outer space visible from the surface of the earth (or any other astronomical object; see Extraterrestrial skies). Birds, insects, airplanes, and kites are often considered to fly in the sky. It is difficult to define precisely for several reasons. During daylight the sky has the appearance of a deep blue surface, but this is the result of the air scattering sunlight. [1] There is no "blue object" above the earth in any normal sense, so it is hard to say what object the sky is. The sky is thus sometimes defined as the denser gaseous zone of a planet's atmosphere. At night the sky has the appearance of a black surface or region scattered with stars. But if we then say that the sky is the entire visible universe, it would not be the same thing we see during the day.

During the day the sun can be seen in the sky, unless covered 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. 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 travelling. The celestial sphere is divided into regions called constellations.

See skies of other planets for descriptions of the skies of various planets and moons in the solar system.

1 Sky luminance and colours
2 Expressions
3 See also
4 References
5 External links

[edit] Sky luminance and colours

The light from the sky is a result of diffuse sky radiation which results in a light blue colour. On a sunny day the earth sky usually looks as a blue gradient — dark in the zenith, light near the horizon (due to Rayleigh scattering). It can turn a multitude of colours such as purple (especially near sunset and sunrise) and black at night.

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 sky^[1].

[edit] Expressions

A "pie in the sky" is something that is wanted very badly but is unobtainable.
When someone's assumptions or premises concerning a particular subject seem to be radically different from everything you know and recognise about the world, you wonder what colour the sky is in their world.
Similarly, because the blue hue of the sky during daytime is such a universally observed and understood phenomenon, people say "as sure as the sky is blue" to mean that something is an indisputable fact.
Blue skies are evoked as a symbol for good times, optimism and emotional peace.
To be sky-clad means to be in the nude.
Similarly, to go skying is a euphemism for stripping or streaking.
To show that there are no limits on what one can discover or accomplish, one can say, "The sky's the limit!"
"Blue sky thinking" means coming up with a host of ideas without being restricted to what is practical (similar to brainstorming); in particular, an optimistic assumption is made that there will be no obstacles or opposition to implementation of the ideas.
"The sky is falling" is a common expression that is used when any sort of disaster is occurring. This came from the fable Chicken Little
"In other news, the sky is up" used to state the obvious.