noun
fine weather; splendid weather
Describes very good, clear, and pleasant weather. Often used in weather forecasts or casual conversation.
今日は上天気で、散歩日和だ。
It's splendid weather today, perfect for a walk.
快晴 is a meteorological term for 'clear weather' with no clouds, whereas 上天気 is a subjective, everyday description of wonderful weather.
Compound of 上 (じょう, 'upper, excellent') and 天気 (てんき, 'weather'), literally 'excellent weather'.