How can sky be plural if there is only one sky?

Answer 1

The plural (skies) is often used to mean "locations up in the air" (the skies over Chicago) and not at the physical blue sky itself. It also refers to the fact that are innumerable appearances to the sky, which determine what you are actually seeing. Seeing clouds could mean a "cloudy sky" although the sky itself is unchanged if you go above the clouds.

Generally, you would use sky and skies interchangeably, except where the physical sky as a whole is concerned, as in "I looked up at the sky" where skies would seem to indicate there was more than one.