"An
atmosphere (from
Greek ἀτμός
(atmos), meaning "vapour", and σφαῖρα
(sphaira), meaning "sphere"
[1][2]) is a layer of
gases surrounding a
planet or other
material body, that is held in place by the
gravity of that body. An atmosphere is more likely to be retained if its gravity is high and the atmosphere's temperature is low."
This is a quote from Wikipedia. It means, a layer of
emotion surrounding a
chorus or other
part of song, that is held in place by the
performance of that body. An atmosphere is more likely to be retained if it's
performance is high and the atmosphere's
feeling is low.
Which suggests, that the way you phrase a certain melody makes it atmospheric, but only if the notes have a right balance between them. About low feeling, it shows that it shouldn't be overplayed, nothing that has too many notes in weird order can sound atmospheric (many types of jazz are good example). It's just gonna be boring. The performance, sound quallity, the quallity of playing live - this has to be high. It has to be better live than on official release.Emotion, feel + right choice and order of notes/chords + a fitting, skilled playing of that idea = atmospheric song. Oh, and about temperature, it should be low, like the original definition says. If you play metal in California, it's gonna sound like the thrash scene from that area, like Slayer, which we kinda saw already. Dark, filthy, energetic yet melodic and inspiring stuff comes from the eternal solitude of northern winter.