If the sky is cloudless, there is not a cloud in it. It looks clear and open; by day it is often bright, and by night you can see the stars. We use it mainly of the sky, or of a day or night (a cloudless afternoon, a cloudless night). It does not mean “no rain later” — only that, right now, the sky is empty of clouds. It is a calm describing word, not slang and not fancy. You will meet it in weather, travel writing, and everyday talk about a fine day.

無雲的、萬里無雲