Three short lines, and a small, odd image at the center: noon with a barb, a sharp point. We usually think of midday as bright and open, but here it seems to prick or sting. The speaker is in a shed, and the poem suggests that the light and heat of noon can't quite reach inside, or can't dissolve what's already there.
The last line holds the real surprise. Even in the middle of the day, the shed holds "the air of night," as if darkness has been stored in the walls and never fully leaves. The words "as ever" make this feel like a habit, something the speaker has noticed again and again.
The mood is quiet, a little uneasy, and watchful. The poem doesn't say whether the speaker is comforted or bothered by this trapped night. Judging by the imagery, the sharp noon outside and the cool dimness within sit side by side, and the speaker stays between them, noticing.