fine weather; good weather
Describes pleasant, clear weather conditions. Often used in forecasts or casual remarks about the weather.
It's fine weather today, isn't it?
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.