rainy-season sky; overcast sky in the rainy season
Refers to the gloomy, cloud-covered sky typical of the Japanese rainy season (梅雨). Often carries a slightly melancholic or oppressive feeling.
Under the rainy-season sky, I have trouble getting the laundry dry.
It's another overcast rainy-season day, and I tend to feel down.
Compound of 梅雨 (つゆ, 'rainy season') and 空 (そら, 'sky'). The reading つゆ for 梅雨 is a sound shift from ばいう, which itself is a borrowing from Chinese.