On a cloudless night, frost may form on horizontal surfaces even when the temperature of the air is above freezing.

在晴朗的夜晚，即使空气的温度在冰点以上，地平面上也能结霜。

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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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The sky is cloudless; the sun shines bright and warm; the songs of birds, and the burst of vitality, fill the air.

碧空如洗，万里无云，阳光温暖而明媚，鸟儿在歌唱，空气中弥漫着勃勃生机。

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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