na-adjective, noun
fine weather; good weather
Describes pleasant, clear weather conditions. Often used in forecasts or casual remarks about the weather.
今日は好天気ですね。
It's fine weather today, isn't it?
好天気に恵まれて、ピクニックを楽しんだ。
Blessed with good weather, we enjoyed the picnic.
天気 is the general word for 'weather', whereas 好天気 specifically means 'fine weather'.
Compound of 好 (good) + 天気 (weather). The reading こうてんき follows the on'yomi of both kanji.