In order to correctly address this question, one has to understand the concept of atmosphere in its literate context. Atmosphere, in literature, "refers to the feeling, emotion, or mood a writer conveys to a reader through the description of setting and objects" (Gentry, study.com)
In Macbeth's soliloquy, the atmosphere is primarily surreal since it contains elements of the supernatural. Macbeth's dagger soliloquy adds to this atmosphere and emphasizes the demonic purpose of Macbeth's quest, which is to commit murder. One can almost feel the dark forces gathering around to urge him toward his foul deed. When he imagines seeing the dagger before him, one senses the chill of malevolence that he himself is experiencing.
Macbeth questions the dagger's appearance and rhetorically asks whether it is real or just a figment of his imagination, brought about by a 'heat oppressed brain.' Clearly, the commission of his crime is what has been uppermost in Macbeth's mind and he is both anxious and afraid of proceeding to what would be the enactment of a most malicious and treacherous act.
The atmosphere becomes suspenseful when Macbeth notices the dagger leading him to Duncan's chamber. He declares that his eyes are made the fools of the other senses or are 'worth all the rest' implying that either his eyes are deceiving him or that they are better attuned to the circumstances than all the rest of the senses put together. His vision is, therefore, sharper and he can clearly envision the probable outcome of what he is about to do. In this regard then, the dagger provides a premonition of what is to come.
This idea is affirmed when Macbeth continues seeing the vision but now it displays gobs of blood on its blade which were absent before. Macbeth strives to erase the vision by declaring that it 'is the bloody business which informs thus to mine eyes.' He accedes that it is his mission to murder the king in his bed that is affecting his mind and causing him to hallucinate.
Macbeth is obviously overwhelmed by the malice of his intended act and he refers to the overwhelming darkness into which he is enfolded, both literally and figuratively. It is a time when evil is afoot and murder has been awakened. He refers to Hecate, the goddess of witchcraft and supreme leader of all witches. He mentions murder, which he personifies, stating that it has been awoken by its sentinel, the wolf, also a creature of darkness, to move secretly through the night towards its target.
These descriptions enhance the foreboding atmosphere and create a mood of malice and pernicious rancor . He alludes to Tarquin, an evil and tyrannical king responsible for the rape of Lucrece. Just as Tarquin moved 'with ravishing strides, towards his design,' does murder now move towards Duncan's chamber to fulfill its fell purpose. Macbeth's allusions are, of course, direct references to himself, for that is exactly what he is about. It is for...
this reason that he asks:
Thou sure and firm-set earth,Hear not my steps, which way they walk, for fearThy very stones prate of my whereabout,And take the present horror from the time,Which now suits with it. Whiles I threat, he lives:Words to the heat of deeds too cold breath gives.
He does not want to be discovered and asks, by way of an apostrophe, that the earth and the cobblestones not to hear his footsteps, for he fears discovery. The time is ripe for him to commit his foul deed for, as long as he only threatens to kill Duncan, the king lives and it cannot be so. Duncan must die. It is this most foul design that creates an atmosphere of surreal and supernatural foreboding.