how does the imagery of the season emphasize the world the boys inhabit

The weather, which is the main indicator and symbol of the general mood on campus, reflects the change that has come over several of the boys. The images of the outdoors become very bleak, depressed, and are conquered by the winter; with "every sprig of vitality snapped" around them, the boys, like Brinker and Finny, become similarly depressed and inactive. The weather seems either to be sympathetic to the boys' moods, or to make a great impression on their feelings.