快晴
かいせい · kaisei · JLPT N2 · noun
good weather
☀️ 快 = quick, 晴 = clear sky: a quick clear sky means good weather
快 means fast or quick, and 晴 means clear, sunny sky. Together they describe the sky clearing up fast into bright, fine weather.
快 — heart radical 忄 + 夬 (phonetic)
快 is a semantic-phonetic compound. The heart radical 忄 carries the meaning, since the original sense referred to a feeling of pleasure or delight. The right side 夬 supplies the sound. The meaning later extended from 'pleased' to 'quick, fast', the dominant modern sense.
晴 — sun 日 + blue/clear 青
晴 is a semantic-phonetic compound. The left element 日 (sun) gives the meaning — clear, sunny weather — while 青 (qīng) on the right supplies the sound. It originally referred to a sky cleared of rain, and still means 'clear/fine weather' today.