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》

It was a cold autumn day with a cloudless, hazy sky and a yellow sun that no longer gave off any heat, the kind you can look at directly without hurting your eyes.

那是个寒冷的秋日，天空朦胧，万里无云，黄色的太阳不再发射出热量，是那种任你直接看都伤不到你的眼睛的太阳。

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The autumn sun sits low in an almost cloudless sky, casting long, attractive shadows.

秋阳低低地挂在几乎没有一丝云的天上，投射下长长的、迷人的影子。

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But the sky was cloudless, and as the sun rose everyone settled down and some even nodded off, which Jake Early regretted, feeling that when the Union soldiers came they should find black folk not at their ease but smartly arrayed as a welcoming company of free men and women.

NPR: E.L. Doctorow on Sherman and 'The March'

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