The sun shone brightly in a cloudless sky.

太阳在无云的天空中明亮地照耀着。

《牛津词典》

In a park setting, the ideal conditions are sun and a cloudless sky.

在公园中时，最好是万里无云的晴天。

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On the verandah the air was scented with the heavy perfume of the tiare, and overhead the Southern Cross shone in a cloudless sky.

露台上，空气里弥漫着蒂阿瑞花的浓郁香气，头顶上，南十字座星在万里无云的天空上闪烁发光。

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Video posted by the rebel forces shows a jet framed in a cloudless sky being shot at, catching fire and falling out of frame.

CNN: STORY HIGHLIGHTS

But there might be an even more animalistic explanation why a cloudless sky buoys us up: "When it's good weather there's much more flesh on show, which cheers most people up, " says Dr Workman.

BBC: Call that a summer?

But a clear, cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more easily than they do red light.

BBC: rainbow