蒸蒸日上 is formed by reduplicating the character 蒸, which conveys rising like steam, to emphasize continuous growth, and then combining it with 日上, where 日 means "day" and 上 means "up", to indicate daily upward progression. This structure directly juxtaposes the intensified sense of flourishing from 蒸蒸 with the temporal and directional aspect of 日上, resulting in a literal depiction of something ascending steadily each day, hence meaning progressively thriving.