perfectly clear and serene sky
A literary, poetic expression describing a sky that is completely clear, calm, and pure. Often used in classical or refined contexts.
On an autumn morning, a perfectly clear and serene sky stretched out.
Her heart was as clear and serene as a cloudless sky.
The phrase combines 真澄 (ますみ), meaning 'truly clear' or 'pure', with 空 (そら, 'sky'). The exact historical derivation is uncertain, but it is a fixed poetic expression.