clear up
晴 centers on weather becoming clear and bright after clouds or rain: the sky clearing up.
晴 combines 日 (sun) with 青, which likely contributes the sound and the idea of clear blue sky. The character expresses the sun appearing and the weather clearing.
The sun 日 and the blue 青 sky come together: when the sun shines in a blue sky, the weather has cleared up.
For セイ, imagine the sun clearing the sky and you say 'say' to the clear day: say -> セイ.
wonderful; splendid; magnificent
clear weather; fine weather
to clear up; to clear away; to be sunny; to stop raining
fine weather (i.e. little or no clouds); fair weather; clear weather; clear sky; fair skies
clear weather; cloudless weather; good weather
to dispel; to clear away; to refresh (oneself)
recreation; diversion; relaxation
splendid; praiseworthy; admirable; brilliant
(scenic) view; prospect; outlook
clear; bright; sunny
break (in the weather, esp. rain or snow); interval of clear weather
(one's) Sunday best; (one's) best clothes; glad rags
splendor; splendour; magnificence; beauty
clear autumnal weather
diversion (from one's worries, troubles, etc.); distraction; amusement
mountain vapor; mountain vapour
bright; cheerful; in high spirits
lookout platform; viewing platform; observation tower; viewpoint; overlook
appearing in one's finest clothes; appearing in one's hour of triumph
clear or rainy weather; sunshine or rain