Atmosphere refers to the sense of a place or moment that the text implies to a reader. It’s often used to refer to ‘atmospheric’ prose which tends to be applied to the moody, broody, and dark, though this isn’t always so. Atmosphere is also often related to the horror genre in reference to the spooky, tense or horrifying because horror often uses an increasing sense of tension to create ‘scares’, although ‘scares’ seems to simplify it a little too much. As horror shows the atmosphere of a piece of prose doesn’t have to be constant. It can become more unsettling, less unsettling, never unsettling, one constant, or specific moments where the atmosphere/mood changes.
Staying on the theme of mood we might suggest that atmosphere and mood can function separately because the mood may be applied to the character’s mood rather than the mood of the piece itself. For instance we can have a comedic story with a grumpy character in which the atmosphere is light but the mood of the character is dark. When we apply mood and atmosphere to the prose it becomes harder to separate the two, the mood affects the atmosphere and the atmosphere affects the mood to the point that they’re almost interchangeable. If we go back to the comedy we can have a story that is usually light and amusing suddenly deal with dark issues and take a dark turn, at this point the characters moods are dark and the atmosphere/mood of the story is dark. This often happens with the death of the character. As we advance from this moment the story can still deal with death but the mood may begin to brighten and become more hopeful even if the atmosphere remains moody. So, perhaps, when we us atmosphere and mood it would be easier to think of it as atmosphere being the sense of the story and mood being the sense of the characters.
This distinction can help when we’re writing scenes because a character’s mood will often impact the atmosphere of a place. Continuing with the comedy example the may be a place linked to the death of a character that is not an inherently grim place but our narrator depicts it as such because they associate this perfectly ordinary place with death. In this case the grim atmosphere is entirely created by the character’s associations not the place itself.
Similarly atmosphere can affect mood; a cheerful character can become afraid when presented with that creepy gothic mansion, or sunlit grove where something doesn’t feel right. In this case the atmosphere is inherent to the place rather than imposed on it by the character. However, we may argue that once again this is not entirely true given the bias against gothic mansions when creating creepy settings.
Given the cliché of the haunted gothic mansion it is possible to create a false atmosphere. People expect the gothic mansion to be creepy and fill in the blanks with their expectations, however, we can reveal this to be inaccurate as we progress and the setting is revealed to be perfectly normal. We can also do this with the information a character gathers about a place. For example, when a character arrives in a new place and is told such-and-such a place is haunted we’re developing a notion of the setting before we see it so by the time the character reaches it we already expect it to be unsettling so the expectations alter the perception. It could turn out there was nothing odd going on there at all.
However, expectations are only one of the tools we can use to create atmosphere.