Atmosphere is the same as mood and both are defined as the emotional feeling generated by a work or a section of a work, as atmosphere/mood can change from scene to scene in a long complex work. Having said that, it is important to note that even if the atmosphere changes between scenes, there is nonetheless usually an overriding atmosphere that remains a prevailing constant throughout the work.
We are first introduced to Arden Forest in II.i in which Duke Senior gives voice to the virtues of the forest. His principal point is that though the forest may at times be a strong adversary to humans' frailty, the forest is nonetheless a faithful friend and adviser revealing truth about a person. This is part of the underlying analogy between the forest and royal court, which is a place where friends may become foes and counselors are dangerous enviers:
Hath not old custom made this [forest] life more sweet
Than that of painted pomp? Are not these woods
More free from peril than the envious court?
Atmosphere, or mood, is set by a combination of such things as diction, setting, the tone of the character (not author's tone) toward his subject, theme, motif, and imagery. In Duke Senior's introduction to Arden, we have an interesting contrast between some of these elements. While the imagery is dark and gloomy, "the icy fang / And churlish chiding of the winter's wind," Duke's tone is bright and happy, "Sermons in stones and good in every thing." While the diction is loaded with punishing adjectives and nouns, peril, envious, penalty, fang, churlish, shrink, ugly, venomous, etc, the theme is optimistic and thankful, custom made this life more sweet / Than that of painted pomp? ... yet a precious jewel ... I would not change it.
In the midst of these contrasts, it is the character's tone that wins out, settles the contrasts, and sets the atmosphere of Arden Forest. While Arden Forest delivers all the vagaries of nature, from cruel icy wind to blazing sun, upon its inhabitants, the forest is a true friend that gives good, joy, peace, and truth. The atmosphere of Arden, then, is optimistic from goodness in truth and sheltering comfort through friendship.
Sweet are the uses of adversity,
Which, like the toad, ugly and venomous,
Wears yet a precious jewel in his head;
And this our life exempt from public haunt
Finds tongues in trees, books in the running brooks,
Sermons in stones and good in every thing.
As confirmation of this, we find the same contrast repeated in Rosalind's and Celia's first encounter with Arden in II.iv. At first, as they enter the forest while in the throes of thirst and hunger, their reaction is none too keen. It is summarized by Touchstone's comment about being a fool for being there for having been in a better place before:
Ay, now am I in Arden; the more fool I; when I was
at home, I was in a better place
Yet, after overhearing Corin and Silvius in earnest talk and after being offered what meager repast Croin can provide them, they immediately see Arden in a different light--a sheltering, comforting light--since they negotiate, in a quick turn in the conversation, to buy Corin's master's sheep farm, flock, and cottage:
And we will mend thy wages. I like this place.
And willingly could waste my time in it.
This confirms the contrasting elements that work together to comprise the atmosphere of Arden: optimistic from goodness in truth and sheltering comfort through friendship.