word-forming element meaning "vapor," from Greek atmos "vapor, steam," from PIE *awet-mo-, from root *wet- (1) "to blow" (also "to inspire, spiritually arouse;" see wood (adj.)).
Entries related to atmo-
1630s, atmosphaera (modern form from 1670s), "gaseous envelop surrounding the earth," from Modern Latin atmosphaera, from Greek atmos "vapor, steam" (see atmo-) + sphaira "sphere" (see sphere). In old science, "vaporous air," which was considered a part of the earth and a contamination of the lower part of the air (n.1).
Þe ouer partye of þe eyr is pure and clene, clere, esy & softe, ffor mevynge of stormys, of wynde and of wedir may nat reche þerto; and so it perteyneþ to heuenlych kynde. And þe neþir partye is nyʒe to þe spere of watir and of erþe, and is troubly, greet and þicke, corpulent and ful of moyst erþy vapoures, as longiþ to erþy partyes. Þe eyr strecchiþ hym kyndely al aboute fro þe ouer partye of þe erþe and of watir anon to þe spere of fire. [Bartholomew Glanville, "De proprietatibus rerum," c. 1240, translated by John of Trevisa c. 1398 ]
First used in English in connection with the Moon, which, as it turns out, practically has none.
'Tis Observed, in the Solary Eclipses, that there is some times a great Trepidation about the Body of the Moon, from which we may likewise argue an Atmo-sphaera, since we cannot well conceive what so probable a cause there should be of such an appearance as this, Quod radii Solares a vaporibus Lunam ambientibus fuerint intercisi, that the Sun-beams were broken and refracted by the Vapours that encompassed the Moon. [Rev. John Wilkins, "Discovery of New World or Discourse tending to prove that it probable there may be another World in the Moon," 1638]
Figurative sense of "surrounding influence, mental or moral environment" is c. 1800.