Sun 日 plus 青 the blue-green of clear depths: 晴 is the sky after rain walks away — sun out, air rinsed, blue restored. The character is a weather report in two strokes-groups: sun, plus the color of clean.
晴 completes the weather trio with 阴 overcast and 雨 rain that every Chinese forecast recites. 晴天霹雳 'thunderbolt from a clear sky' is shocking news at its most idiomatic; poets since Su Shi have argued West Lake is 晴方好 — best on a clear day (he then argued rain was equally good; poets hedge).
Sun 日 dyeing the sky blue-green 青 — a CLEAR day.
周杰伦's ballad《晴天》made this weather character a generation's shorthand for school-days nostalgia — millions learned to write it from a lyric video.