when weather turns warm and humid due to southerly airstream, affecting the southern provinces of China during Spring
Definitions (粵典–words.hk)
名詞
一種華南嘅氣候現象；大概春天嘅時候，來自太平洋帶有大量水汽嘅南風吹向中國大陸，令天氣開始回暖同濕度大幅上升，甚至會出現傢私、地板、牆壁出水嘅情況 continuous warm and wet weather between spring and summer in southern China; literally "return south"
今日回南，咩衫都唔乾。
今日回南，咩衫都唔干。
gam1 jat6 wui4 naam4, me1 saam1 dou1 m4 gon1. It is starting to "return south" today, it's so humid that washed clothes do not dry.
參看
大回南、回南
Definitions (Wiktionary)
noun, Cantonese, Xiamen, Zhangzhou-Hokkien
warm and humid weather (in the rainy season in southern China) (syn.) 水南天, 南风天, 南風天, 回南