Atmosphere (or mood) refers to the feeling the reader gets about the story based on the details the author uses. Atmosphere is created by the description of backgrounds and settings, and sometimes by description of the characters and events.
The atmosphere is the feeling the author wants the reader to pick up subconsciously - descriptions of cloudy skies, rain, and bitter winds make the reader feel sad and depressed, making a story seem dark and dreary; descriptions of bright green fields of grass, warm summer breezes, and sunny skies make the reader feel happy and relaxed, making a story seem bright and cheerful.
Atmosphere is created mainly in two ways: through word choice and through sentence structure.
How an author describes setting affects how the reader feels as they read the story. For example, if you're writing about a large house, how you describe it changes the reader's feelings and expectations about what is going to happen:
The stately mansion gleamed under a fresh coat of white paint. A giant window over the double-door entry let light in and let the sparkling reflections from the chandelier in the foyer dazzle those who came up the crushed-stone driveway.
The old mansion's paint was peeling away; the once-white paint had mildewed and faded to a slushy gray. The giant window over the double-door entry was cracked and partially missing; a large shard jutted up from the bottom of the frame like a single jagged tooth. The doors themselves hung slightly askew and looked as though they'd tried to keep out a pack of wild, scratching dogs, but had failed.
Here's a sample of a story that starts with an ominous and scary atmosphere. See if you can spot some of the words that create the dark atmosphere.
Try to come up with a list of words that have happy, positive connotations to them. Then try coming up with a list of words that have sad or depressing connotations.
As an example, if you are trying to make a scene seem to move quickly to the reader, stick to using very few subjects and lots of verbs; longer sentences with less punctuation will add to the sense of quickness.
John leaped through the doors, wondering where the bomb could be, searching quickly with his eyes. Not spotting it, he dashed through the room, leaping over the fire raging on the rug, landing in a somersault so he could keep moving forward.
Notice: Verbs that end in "-ing" make the reader feel like the action is happening at that moment; past-tense verbs slow the action down because it's already happened.
Also, there is very little detail to the setting- the descriptions are of the actions, not of the objects.
Try writing a very brief action scene - no more than a few sentences. Keep the action moving!
Similarly, you can make a scene seem slower either by using lots of shorter sentences with plenty of end punctuation, or by going into in-depth descriptions.
John walked slowly. Ahead was the pond. It shimmered under the sun. He smiled. He knew he would get there eventually.
Lots of end punctuation make the reader slow down. The main character only does one major action in all of these sentences; the rest are just describing.
As John slowly ambled along the crushed-stone path that wound its way leisurely through the park, he couldn't help smiling as he remembered how Molly's face had lit up when they first met.
This very long sentence is basically saying, "John walked." The rest is detailed description of the setting. This helps the reader picture what the setting is like, but it also prevents major action from happening very quickly.
Try writing a very brief scene with a happy and positive atmosphere using both word choice and sentence structure.
REMEMBER: Atmosphere is very different than the tone of a piece.