clear up
霽 centers on weather becoming clear after rain or clouds: the sky clearing up.
霽 combines 雨 (rain) with 齊, which likely contributes the sound and the idea of evenness or completion, suggesting rain ending and the sky becoming clear.
Rain 雨 stops and the sky becomes even and clear like the orderly arrangement of 齊 below.
For セイ, imagine the sky clearing up after rain, and you say 'say' to the clear sky: say -> セイ.