Sky blue refers to a collection of shades comparable to that of a clear daytime sky. Typically it is a shade of cyan or light teal, though some iterations are closer to light azure or light blue. The term (as "sky blew") is attested from 1681. A 1585 translation of Nicolas de Nicolay's 1576 Les navigations, peregrinations et voyages faicts en la Turquie includes "the tulbant [turban] of the merchant must be skie coloured".
Displayed at right is the web colour sky blue.
An atmosphere is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. The name originates from Ancient Greek ἀτμός (atmós) 'vapour, steam' and σφαῖρα (sphaîra) 'sphere'. An object acquires most of its atmosphere during its primordial epoch, either by accretion of matter or by outgassing of volatiles. The chemical interaction of the atmosphere with the solid surface can change its fundamental composition, as can photochemical interaction with the Sun. A planet retains an atmosphere for longer durations when the gravity is high and the temperature is low. The solar wind works to strip away a planet's outer atmosphere, although this process is slowed by a magnetosphere. The further a body is from the Sun, the lower the rate of atmospheric stripping.
Aside from Mercury, all Solar System planets have substantial atmospheres, as does the dwarf planet Pluto and the moon Titan. The high gravity and low temperature of Jupiter and the other gas giant planets allow them to retain massive atmospheres of mostly hydrogen and helium. Lower mass terrestrial planets orbit closer to the Sun, and so mainly retain higher density atmospheres made of carbon, nitrogen, and oxygen, with trace amounts of inert gas. Atmospheres have been detected around exoplanets such as HD 209458 b and Kepler-7b.
Diffuse sky radiation, is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, the determinative process for changing the colors of the sky. It is normally measured on a horizontal surface, thus frequently termed diffuse horizontal irradiance (DHI), often in the unit of watts per square meter (W/m). Approximately 23% of direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere; of this amount (of incident radiation) about two-thirds ultimately reaches the earth as photon diffused skylight radiation.
The dominant radiative scattering processes in the atmosphere are Rayleigh scattering and Mie scattering; they are elastic, meaning that a photon of light can be deviated from its path without being absorbed and without changing wavelength.
Azure is the color between cyan and blue on the spectrum of visible light. It is often described as the color of the sky on a clear day.
On the RGB color wheel, "azure" (hexadecimal #0080FF) is defined as the color at 210 degrees, i.e., the hue halfway between blue and cyan. In the RGB color model, used to create all the colors on a television or computer screen, azure is created by adding a 50% of green light to a 100% of blue light.
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gemstone for millennia due to its hue.
The robin egg blue or sky blue color of the Persian turquoise mined near the modern city of Nishapur, Iran, has been used as a guiding reference for evaluating turquoise quality.
Chappuis absorption (French: [ʃapɥi]) refers to the absorption of electromagnetic radiation by ozone, which is especially noticeable in the ozone layer, which absorbs a small part of sunlight in the visible portion of the electromagnetic spectrum. The Chappuis absorption bands occur at wavelengths between 400 and 650 nm. Within this range are two absorption maxima of similar height at 575 and 603 nm.
Compared to the absorption of ultraviolet light by the ozone layer, known as the Hartley and Huggins absorptions, Chappuis absorption is distinctly weaker. Along with Rayleigh scattering, it contributes to the blue color of the sky, and is noticeable when the light has to travel a long path through the Earth's atmosphere. For this reason, Chappuis absorption only has a significant effect on the color of the sky at dawn and dusk, during the so-called blue hour. It is named after the French chemist James Chappuis (1854–1934), who discovered this effect.