An Etymological Dictionary of Astronomy and Astrophysics
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1) The gaseous envelope surrounding a star, planet, or moon. Several solar system planets retain considerable atmospheres, due to their strong gravitational force. The gas motions in the planetary atmosphere, as a response to the heating, coupled with the rotation forces, generate the meteorological systems. The planetary satellites → Titan and → Triton also have atmospheres (M.S.: SDE).
2) A unit of pressure, called standard atmosphere, which is the pressure of air balanced by a column of mercury 76 cm high with a density of the mercury of 13.595 g/cm³ at normal acceleration of gravity. Such a column applies a pressure equal to its weight to each square cm, or 1.01325 x 10⁶ dynes/cm² = 1.01325 x 10⁵ N/m². Since this pressure is equal to 1.03323 kilograms of force per square centimeter, instead of it use is often made of the technical atmosphere (at), exactly equal to 1 kgf/cm².

New L. atmosphaera, from Gk. atmos "vapor" + spharia "sphere."

Havâsepehr, from Mod.Pers. havâ, → air, + sepehr, → sphere. Javv "air, atmosphere," from Ar. jauw.

The envelope of → gas and → dust around a → comet nucleus, also known as → coma. As the comet approaches the → Sun, the frozen materials → sublimate and give rise to an expanding atmosphere. The atmosphere is composed of dust, → molecules, → radicals, and molecular → ions released from the inner coma with velocities ~ 0.5 to 1 km s^-1, well above the → escape velocity for the nucleus. The → chemical species observed in cometary spectra can be divided into several categories: (i) atoms and molecules related to → water (H, O, OH, OH⁺, H2O, H2O⁺), (ii) carbon and related molecules (C, C⁺, CO, CO⁺, CO₂⁺, C₂, CH, CH⁺, HCO, H₂CO), (iii) → nitrogen and related molecules (CN, CN⁺, HCN, CH₃CN, NH, NH₂, N₂⁺, NH₃, NH₄), (iv) → sulphur and related molecules (S, CS, S₂, H₂S⁺), (v) → metals (Na, K, Ca, Co, Cr, Cu, V, Fe, Mn, Ni). For a typical average comet the neutral atmosphere is first seen when the heliocentric distance is d ≤ 3 → astronomical units.

→ cometary; → atmosphere.

That part of the atmosphere where the effects of the ground on the → turbulence conditions are negligible.

→ free, → atmosphere.

A simplifying assumption in the models of stellar atmosphere, according to which the absorption coefficient has the same value at all wavelengths.

→ gray; → atmosphere.

The gaseous envelope surrounding Jupiter. It is about 90% → hydrogen and 10% → helium (by numbers of atoms, 75/25% by mass) with traces of → methane, → water, and → ammonia. This is very close to the composition of the primordial → solar nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. The outermost layer is composed primarily of ordinary → molecular hydrogen and helium. Visually, Jupiter is dominated by two atmospheric features; a series of ever-changing atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the → Great Red Spot.

→ Jupiter; → atmosphere.

Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the → troposphere and lower → stratosphere); hence used in contrast to the common meaning for the → upper atmosphere. In other contexts, the term implies the lower troposphere (Meteorology Glossary, American Meteorological Society).

→ lower; → atmosphere.

The region lying between the → troposphere and the → thermosphere comprising the → stratosphere and the → mesosphere (Meteorology Glossary, American Meteorological Society).

→ middle; → atmosphere.

An approximation used in many stellar atmosphere models that depict the atmosphere as being only one-dimensional and bounded at the top and bottom by horizontal plane surfaces normal to the direction of gravity.

→ plane; → parallel; → atmosphere.

1) An atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidating gasses or vapours. Usually nitrogen or hydrogen gas is used in order to produce specific effects, e.g. on ceramic wares being fired.
2) An atmosphere of a planet or moon which has a high hydrogen content, either in the form of free hydrogen or hydrogen-containing compounds, such as methane or ammonia. The early atmosphere of Earth is thought to be reducing, dominated by carbon dioxide.

Reducing verbal adj. of → reduce; → atmosphere.

An atmosphere of a planet that forms after primordial gases had been lost or had failed to accumulate. A secondary atmosphere develops from internal volcanic outgassing, or by accumulation of material from comet impacts. It is characteristic of terrestrial planets, such as Earth, Mercury, Venus, and Mars. → primordial atmosphere.

→ secondary; → atmosphere.

A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.

→ standard; → atmosphere.

The outer envelope of gas and plasma that surrounds a star; characterized by pressure, temperature, density, chemical composition, and opacity at varying altitudes.

→ stellar; → atmosphere.

A model that computes the radiation field crossing the boundary layers of a star at all frequencies. The parameters used for the characterization of a stellar atmosphere model are: → effective temperature, → surface gravity, and → metallicity.

→ stellar; → atmosphere; → model.

The general term applied to the atmosphere above the → troposphere.

→ upper; → atmosphere.