The brightness and color of the sky vary greatly over the course of a day, and the primary cause of these properties differs as well. See also: Sky brightness, Earth's shadow, and Chappuis absorption Recent developments relate to "all sky models" for modelling sky luminance under weather conditions ranging from clear to overcast. Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. An extreme example is the Moon, where no atmospheric scattering occurs, making the lunar sky black even when the Sun is visible.
At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower air density. The effect is not very obvious on clear days, but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight. Distant clouds or snowy mountaintops may appear yellowish. The Sun is not the only object that may appear less blue in the atmosphere. As the air mass nears infinity, scattered daylight appears whiter and whiter. Red light is also scattered if there is enough air between the source and the observer, causing parts of the sky to change color as the Sun rises or sets. Scattered light from the horizon travels through as much as 38 times the air mass as does light from the zenith, causing a blue gradient looking vivid at the zenith and pale near the horizon. Scattering effects also partially polarize light from the sky and are most pronounced at an angle 90° from the Sun. The sky can turn a multitude of colors such as red, orange, purple, and yellow (especially near sunset or sunrise) when the light must travel a much longer path (or optical depth) through the atmosphere. 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. Individual water droplets refract white light into a set of colored rings. Scattering also occurs even more strongly in clouds. The remaining direct sunlight, having lost some of its shorter-wavelength components, appears slightly less blue. Scattering is significant for light at all visible wavelengths, but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source: the Sun.
The scattering due to molecule-sized particles (as in air) is greater in the directions both toward and away from the source of light than it is in directions perpendicular to the incident path. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.Įxcept for direct sunlight, most of the light in the daytime sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh scattering. Certain birds and insects, as well as human inventions like aircraft and kites, can fly in the sky. Lightning and precipitation are also visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. At night, the Moon, planets, and stars are similarly visible in the sky. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by clouds. The night sky appears to be a mostly dark surface or region spangled with stars. The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light). In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the sky bowl) appearing flatter during the day than at night. Usually, the term sky informally refers to a perspective from the Earth's surface however, the meaning and usage can vary. The celestial sphere is conventionally divided into designated areas called constellations. This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting. In the field of astronomy, the sky is also called the celestial sphere. The sky is the panorama obtained from observing the universe from the Earth and other celestial bodies' surface.