回南天通常指每年冬春季（2-4月居多）由于强盛的西南暖湿气流引起的气温回暖、温度回升、进而使墙壁地面甚至物品表面出现水汽的温暖返潮的天气现象。

"Hui nan tian" is a meteorological phenomenon which is caused by the combination of cold and warm air currents, bringing unbearable humidity and moisture between late spring and early summer. The phenomenon typically occurs during February and March, marking the resurgence of humidity in the spring.

一般来说，“回南天”的形成需要两个条件：1. 有长时间的低温，日平均气温低于12℃，且至少要持续3天以上。2. 暖湿气流遇到冰冷物体，物体表面容易产生水珠。

Typically, the formation of "hui nan tian" requires two conditions: First, prolonged low temperatures, with the daily average temperature below 12°C, lasting for at least three consecutive days; second, warm and moist air currents encountering cold objects, causing water droplets to form on the surface.

来源：中国日报双语新闻