Enhancing the creepy feeling
Describing the weather and external conditions certainly does this effectively. The whole story has a masterfully built atmosphere of oppressive dread - not a dread linked to fear of some specific event, but a deep, existential disquiet. The countenance and behaviour of Roderick Usher, the mostly behind-the-scenes sickness and death of his sister, and the lightless and gloomy atmosphere within the house all serve to heighten this feeling, of course, but it's already introduced in the first paragraphs of the story, before the narrator even enters the house, by the description of the external atmosphere.
Here's the opening sentence of the story:
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.
The word choice here is already evoking the ambience that pervades the whole story: the day is "dull" and "dark", the clouds "oppressive", the landscape "singularly dreary", the house "melancholy" (more on this later), and of course the time of day is evening, the most depressing hour, as the light fades and darkness closes in. Already, then, we're starting to feel that gloom and dread which makes the story so memorable. This is setting the scene for what is to come later. It's no surprise when the second sentence is:
I know not how it was -- but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit.
Imagine if, instead, the narrator had ridden through verdant countryside in bright sunshine with birdsong all around, and arrived at the House of Usher around midday. If we'd seen that in the first sentence, then the second would have been an unexpected shock, and the writer would have had to work harder to instil in us that feeling of oppressive gloom. He could still have done it, of course, using much the same description of the interior and inhabitants of the house, but it would have taken longer, and might have been lessened by the memory of the pleasant exterior remaining in our minds.
Then again later on, towards the end of the story, we get a vivid description of the storm:
The impetuous fury of the entering gust nearly lifted us from our feet. It was, indeed, a tempestuous yet sternly beautiful night, and one wildly singular in its terror and its beauty. A whirlwind had apparently collected its force in our vicinity; for there were frequent and violent alterations in the direction of the wind; and the exceeding density of the clouds (which hung so low as to press upon the turrets of the house) did not prevent our perceiving the life-like velocity with which they flew careering from all points against each other, without passing away into the distance. I say that even their exceeding density did not prevent our perceiving this -- yet we had no glimpse of the moon or stars -- nor was there any flashing forth of the lightning. But the under surfaces of the huge masses of agitated vapour, as well as all terrestrial objects immediately around us, were glowing in the unnatural light of a faintly luminous and distinctly visible gaseous exhalation which hung about and enshrouded the mansion.
This is a sign that things are escalating: the atmosphere has shifted from a quiet, pervading dread to a frantic, hysterical terror. We've already seen the change in Roderick Usher's demeanour, the increase in his symptoms and excitement, and even the narrator being affected enough for insomnia. With this description of the storm outside, we get the feeling that the story has just shifted up another gear. This prepares us for the terror of Madeline's reappearance at the climax.
Again, imagine if instead the weather outside was as oppressively still as at the start of the story, the air dull and heavy and soundless. In that case, we might have been slower to accept how significantly the condition of both main characters has changed. The heightening terror throughout the reading of the Ethelred story would have been (slightly) lessened, since the weather is constantly mentioned or alluded to in subtle ways throughout these paragraphs - we're never permitted to forget it.
The link between the building and the family
I noticed these sentences while rereading the story just now, and at first didn't think much of them:
I had learned, too, the very remarkable fact, that the stem of the Usher race, all time-honoured as it was, had put forth, at no period, any enduring branch; in other words, that the entire family lay in the direct line of descent, and had always, with very trifling and very temporary variation, so lain. It was this deficiency, I considered, while running over in thought the perfect keeping of the character of the premises with the accredited character of the people, and while speculating upon the possible influence which the one, in the long lapse of centuries, might have exercised upon the other -- it was this deficiency, perhaps, of collateral issue, and the consequent undeviating transmission, from sire to son, of the patrimony with the name, which had, at length, so identified the two as to merge the original title of the estate in the quaint and equivocal appellation of the "House of Usher" -- an appellation which seemed to include, in the minds of the peasantry who used it, both the family and the family mansion.
But I now believe that this apparently throwaway remark, from the narrator's internal monologue in the third paragraph, is more significant than it seems. The building symbolises the family, and we can see this connection in many different ways throughout the story.
- Firstly, the title. "The Fall of the House of Usher" could refer either to the downfall of the family and the deaths of its last surviving scions, or to the literal collapse of the house at the end of the story.
- The gloomy exterior of the house, which the narrator spends several paragraphs dwelling on at the start, matches perfectly the condition of both Usher siblings. The house is described as bleak and depressing, removed from the heavens above by its vaporous atmosphere; Roderick Usher is described as wan, cadaverous, and presumably similarly removed from his surroundings (at least no mention is made of him venturing outside); and both siblings are afflicted by a sort of morbid apathy which he describes as "a constitutional and a family evil".
- The unnatural storm which gathers around the house towards the end of the story coincides with the height of Roderick Usher's distractedness and terror. His hysteria mirrors that of the weather - which, given that it seems to be an extremely localised phenomenon, can be seen as an attribute of the house more than its wider surroundings.
- The deaths of both Usher siblings, and thus of the line of the Usher family, are followed almost immediately by the utter destruction of the building, swallowed up by the eerie tarn.
So the description of the external atmosphere surrounding the house, as well as being a writing technique to evoke the appropriate feeling in the reader, is also meant to be a direct in-story correspondence with the mental atmosphere surrounding the Usher siblings. Just as they are oppressed by their own mental illness and drowning in unnatural despair, so the house is oppressed by heavy clouds and ends up drowning in the tarn. The parallel is clear.