It needs cloudless sky to be really accurate.

Well how many cloudless skies do you think there were above Central Europe between 1940 and 1945?

China News Service, Beijing, October 1 (reported by Yin Li) A cloudless blue sky in the day and the red clouds at evening......Recently, Beijing kept in the mode of the most beautiful blue sky. Thousands of Beijing residents couldn't help to change tactics to show their pictures on social platforms and constantly make status updates to show their happinesses. They said that the most beautiful skyline with a blue sky and white clouds was the best business card of Beijing .