The first image that leads to the gloomy atmosphere of the poem appears in the first line, where the speaker describes a “midnight dreary.” Both words evoke gloom. Even the words “many a quaint and curious volume of forgotten lore” are kind of creepy and make you think of a spooky story.
Referring to the room as a “chamber” adds to the spooky atmosphere. The word just sounds so much gloomier than some simple word like “room.” The images continue as the speaker says it’s a “bleak December” because bleak is a gloomy word and December is a cold, dead, winter month. Consider these lines from the second stanza.
And each separate dying ember wrought its ghost upon the floor.
Eagerly I wished the morrow; - vainly I had sought to borrow
From my books surcease of sorrow - sorrow for the lost Lenore (stanza 2)
Even before the Raven really appears (and a raven is a gloomy bird), there is plenty of gloom and doom as a lonely student sits up mourning a lost love.
Then this ebony bird beguiling my sad fancy into smiling,
By the grave and stern decorum of the countenance it wore,
`Though thy crest be shorn and shaven, thou,' I said, `art sure no craven.
Ghastly grim and ancient raven wandering from the nightly shore – (stanza 8)
When the bird appears, more gloomy images come with it. The word “ebony” and the black bird make you think of gloom. Words like “grave” and “grim” do as well.