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The sky is often defined as the place a person sees when he or she looks up from the earth. Although almost everyone has seen it, the sky is hard to be defined precisely. The concept of the sky, as it is applied here on Earth, can be generalized to mean the hemisphere above the horizon, or above the visible surface of the planet and objects attached to it. It can be seen when one looks upward from the surface of a planet (see Skies of other planets). The sky is also sometimes defined as the denser gaseous zone of a planet's atmosphere.

The color of the sky is a result of diffuse sky radiation. 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 turns orange and red during sunrise and sunset, and becomes black at night. During the day the Sun can be seen in the sky, unless covered by clouds. During the night (and to some extent during the day) Moon and stars are visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbowgardens at the Canada pavilion at Epcot in Walt Disney World, Lake Buena Vista, Florida, United States. A rainbow is an optical and meteorological phenomenon that causes a (nearly) continuous spectrum of light to appear in the sky when the sun shines ontos, and aurorae. LightningFor alternate meanings, see Lightning (disambiguation). Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. Lightning's abrupt electric discharge is accompanied by the emission of light. The electricity passing through can also be seen in the sky during stormEnschede, The Netherlands. A storm is a kind of severe weather marked by hard wind (a wind storm), thunder and lightning (a thunderstorm), heavy precipitation such as ice ( ice storm), or wind transporting some substance through the atmosphere (as in a dus. As a result of human activities, smogFor the rock band named Smog see Smog (band). Victorian-era London was notorious for its thick smogs, or 'pea-soupers', a fact that is often recreated to add an air of mystery to a period costume drama. Smog is a kind of heavy air pollution. Under special during the day and light radiance during the night are often seen above large cities. People also say that birdFor other meanings of bird see bird (disambiguation). Many see text Birds are bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. There are almost 9000 known species of birds ins, airplanes, and kiteKite flying is the hobby of flying kites. A kite is an object that flies by opposing the force of the wind with the tension of a string held by the operator. See types of kites, below, for a list of different types and styles. Those flown by American chils fly in the sky.

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 in. 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.