Aozora versus Aoi sora


Can we use 青空 and 青い空 interchangeably?

Is there any difference between the two? Which is the most natural?

Thank you!




In this specific case, yes, 青空 and 青い空 both mean "blue sky". When used on
their own, there is no surprising connotation and thus they are
interchangeable. But 青空～ in a compound noun can mean "open-air" or "outdoor"
(e.g., [青空教室](https://jisho.org/word/%E9%9D%92%E7%A9%BA%E6%95%99%E5%AE%A4) is
not the same as 青い空教室).

Note that it's not always true for similar pairs.
[白紙](https://jisho.org/word/%E7%99%BD%E7%B4%99) is not the same as 白い紙, and
[赤本](https://jisho.org/word/%E8%B5%A4%E6%9C%AC) is not the same as 赤い本.




In this specific case, yes, 青空 and 青い空 both mean "blue sky". When used on
their own, there is no surprising connotation and thus they are
interchangeable. But 青空～ in a compound noun can mean "open-air" or "outdoor"
(e.g., [青空教室](https://jisho.org/word/%E9%9D%92%E7%A9%BA%E6%95%99%E5%AE%A4) is
not the same as 青い空教室).

Note that it's not always true for similar pairs.
[白紙](https://jisho.org/word/%E7%99%BD%E7%B4%99) is not the same as 白い紙, and
[赤本](https://jisho.org/word/%E8%B5%A4%E6%9C%AC) is not the same as 赤い本.

