Sky

This article is about Earth's sky. For other skies, see extraterrestrial skies. For other uses, see Sky (disambiguation).

Crepuscular rays of light shining through clouds near the Washington Monument in Washington D.C.

The sky, also known as the celestial dome]]], 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. 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 pollution during the night are often seen above large cities.

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.

During the day

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: Atmospheric optics

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.

Dusk and dawn

Amount of air still illuminated after sunset, at the horizon. Normalized so that zenith is 1 airmass

Main articles: Earth's shadow and Sky brightness

The intensity of the sky varies greatly over the day and the primary cause differs as well. During daytime when the sun is above the horizon direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. In twilight, the period of time between sunset and sunrise, the situation is more complicated and a further differentiation is required.

The Earth's shadow is the shadow that the Earth itself casts on its atmosphere, which is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise. When the weather conditions and the observer's viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band. Assuming the sky is clear, the Earth's shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band. A related phenomenon is the "Belt of Venus" or "anti-twilight arch" (sometimes written as "antitwilight arch"), a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. There is no clear dividing line between the Earth's shadow and the Belt of Venus; one colored band shades into the other in the sky.

Twilight is divided into three segments according to how far the sun is below the horizon in segments of 6°. After sunset the civil twilight sets in, and ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the sun reaches heights of -6° and -12°, after which comes the astronomical twilight defined as the period from -12° to -18°. When the sun drops more than 18° below the horizon the sky generally attains its minimum brightness.

Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.

During the night

The Milky Way can be seen as a very large streak or arc across the night sky

Main article: Night sky

The term night sky refers to the sky as seen at night. The term is usually associated with skygazing and astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing. The fact that the sky is not completely dark at night can be easily observed. Were the sky (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky.

The night sky and studies of it have a historical place in both ancient and modern cultures. In the past, for instance, farmers have used the state of the night sky as a calendar to determine when to plant crops. The ancient belief in astrology is generally based on the belief that relationships between heavenly bodies influence or convey information about events on Earth. The scientific study of the night sky and bodies observed within it, meanwhile, takes place in the science of astronomy.

The visibility of celestial objects in the night sky is affected by light pollution. The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas.

Use within weather forecasting

Marestail shows moisture at high altitude, signalling the later arrival of wet weather.

See also: Weather forecasting

Along with pressure tendency, the condition of the sky is one of the more important parameters used to forecast weather in mountainous areas. Thickening of cloud cover or the invasion of a higher cloud deck is indicative of rain in the near future. At night, high thin cirrostratus clouds can lead to halos around the moon, which indicates an approach of a warm front and its associated rain. Morning fog portends fair conditions and can be associated with a marine layer, an indication of a stable atmosphere. Rainy conditions are preceded by wind or clouds which prevent fog formation. The approach of a line of thunderstorms could indicate the approach of a cold front. Cloud-free skies are indicative of fair weather for the near future. The use of sky cover in weather prediction has led to various weather lore over the centuries.

Tropical cyclones

Picture of the sky within the eye of a tropical cyclone

Within 36 hours of the passage of a tropical cyclone's center, the pressure begins to fall and a veil of white cirrus clouds approaches from the cyclone's direction. Within 24 hours of the closest approach to the center, low clouds begin to move in, also known as the bar of a tropical cyclone, as the barometric pressure begins to fall more rapidly and the winds begin to increase. Within 18 hours of the center's approach, squally weather is common, with sudden increases in wind accompanied by rain showers or thunderstorms. Within six hours of the center's arrival, rain becomes continuous. Within an hour of the center, the rain becomes very heavy and the highest winds within the tropical cyclone are experienced. When the center arrives with a strong tropical cyclone, weather conditions improve and the sun becomes visible as the eye moves overhead. Once the system departs, winds reverse and, along with the rain, suddenly increase. One day after the center's passage, the low overcast is replaced with a higher overcast, and the rain becomes intermittent. By 36 hours after the center's passage, the high overcast breaks and the pressure begins to level off.

Significance in mythology

Jupiter, Ancient Roman sky deity

See also: Astrology and Sky father

Many mythologies have deities especially associated with the sky. The Egyptian god Horus was the god of the Sky. Zeus was the god of the sky and thunder in Greek mythology. The Roman god Jupiter was the god of sky and thunder. Many cultures have drawn constellations between stars in the sky, using them in association with legends and mythology about their deities.

See also

• Cyanometer

External links

• Day sky images
• Night Sky images
• Sky photo gallery
• Why is the sky blue?