Meaning:

Literally meaning “the return of the southern dampness,” this term describes the humid season in southern China that typically occurs in spring. As the season sets in, warmer temperatures and a significant increase in humidity are noticeable. People often remark that it feels hotter outside than inside. Mirrors, glass windows, and steel doors become coated with fine droplets of water. Floors grow damp and slippery, while laundry hung out to dry seldom completely dries. These days are seen as a major annoyance, only alleviated by a strong northern cold front or a continued rise in temperature.

Example:

A: 怎么晾在阳台上的衣服三天还干不了？

Zěnme liàng zài yángtái shang de yīfú sāntiān hái gānbuliǎo？

It’s been three days, yet the clothes I’ve hung out on the balcony are still damp.

B: 因为回南天到了。

Yīnwèi huínántiān dào le。

Ah, the humid season has arrived.