Poe believed that the point of writing poems or stories was to create an emotional response in the reader. That means that his fiction is less concerned with realism, per se, than it is with “atmospherics,” as you say. Take for example the following passage from beginning of “The Fall of the House of Usher”:
DURING the whole of a dull, dark, and soundless day in the autumn of the year, when the clouds hung oppressively low in the heavens, I had been passing alone, on horseback, through a singularly dreary tract of country, and at length found myself, as the shades of the evening drew on, within view of the melancholy House of Usher. I know not how it was—but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit. I say insufferable; for the feeling was unrelieved by any of that half-pleasurable, because poetic, sentiment, with which the mind usually receives even the sternest natural images of the desolate or terrible. I looked upon the scene before me—upon the mere house, and the simple landscape features of the domain—upon the bleak walls—upon the vacant eye-like windows—upon a few rank sedges—and upon a few white trunks of decayed trees—with an utter depression of soul which I can compare to no earthly sensation more properly than to the after-dream of the reveller upon opium—the bitter lapse into every-day life—the hideous dropping off of the veil.
The effect of the passage is to create an emotion – one of oppression, or dread – in the reader. The way he does it is partly through the use of descriptive language the “dull, dark, soundless day,” the “singularly dreary” countryside the traveller crosses – we can visualize, so an extent, this place, with its “vacant eye-like” windows, and “white trunks of decayed trees.” But key element in Poe’s ability to evoke an emotional response lies often in what he does not say explicitly. When he says the house was “insufferable,” it is because of what it lacks – the “half-pleasurable, because poetic, sentiment” one “usually” has when looking upon the melancholy. What that sentiment might be, only the reader really can say. That is, Poe sets the stage with his description, but leaves the actual emotion evoked up to the reader. The effect is intense because it is internalized.
There are many other examples. In “The Tell Tale Heart,” the tension in the story works off of a form of dramatic irony: the narrator can hear the beating heart of the murdered old man, but can the police? As readers, we are put in the murderer’s shoes, and his mania becomes our own. In “The Pit and the Pendulum,” the narrator deliberately refuses to open his eyes; his imagining of his surroundings, and his effort to reconstruct his memory of events parallels the reader’s own effort to comprehend events. In “The Raven,” the repeating word, “nevermore,” certainly evokes a melancholy feeling, one which the poem itself does not define precisely, instead leaving a blank space for the reader to fill with his own emotion.