We may expect muggy weather when the rainy season begins.

雨季开始时，我们预料有闷热的天气。

《新英汉大辞典》

The situation became worse for them when the rainy season set in.

雨季开始后，他们的情况更糟了。

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Though conditions are extremely difficult, Geddo says many people he met told him they are staying put in the hopes they can plant a crop when the rainy season begins in October.

他说，尽管他们面临的情势极度艰难，他接触到的许多人还是对他说，他们留下不动，指望十月份的雨季开始时，可以种些庄稼糊口。

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When swollen by the rainy season, the river will cascade through the turbines, turning out up to 11, 200 megawatts adding 10% to Brazil's existing generating capacity.

ECONOMIST: Energy in Brazil

The diving is phenomenal year-round, but the crossing to the reef can be rough in the peak rainy season (July to September) and from December to February when the northeast monsoon winds are at their strongest.

BBC: The undiscovered jewel of the Philippines

In the rainy, summer season, when the tourists are gone and the temperature soars, he devotes himself full time to the cattle side of his business.

FORBES: Notes From A Distant Node