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You would think we never use "skies" since there's only one sky, but I've heard it often enough that I often want to say "skies" rather than "sky". The problem is that I never gave much thought about it, so I have no idea why I even want to do that.
When do we say “skies” instead of “sky”?
Wordnik gleans from a few dictionaries and puts all the definitions on one page. Some of the definitions of sky found there include:
sky (n.) n. The appearance of the upper atmosphere, especially with reference to weather. Often used in the plural: Threatening skies portend a storm.
sky (n.) The apparent arch, or vault, of heaven, which in a clear day is of a blue color; the heavens; the firmament; -- sometimes in the plural.
(emphasis added)
These entries show that a plural form is acceptable, as you've noticed; however, they don't delve into when might be the right time to use the word skies. The best I could find about that was in Cambridge:
skies [ plural ] the sky in a particular state or place:
For weeks we had cloudless blue skies.
We're off to the sunny skies of Florida.
Near as I can tell, it's okay to use the plural form when you are referring to the weather, or to the sky in a particular state or place. That said, the plural wouldn't be mandatory in these cases (although you may have to tweak the wording); these sentences are also grammatical and idiomatic:
As for this one:
I think skies sounds more natural there, but I don't think I can deem the singular as ungrammatical.
skies can refer to the sky over a period of time during which the sky changes.
Tomorrow we can expect mostly cloudy skies.
The underlying idea, I think, is of the changing sky.
The plural is also used with the ocean:
We sailed to warmer waters.
where the underlying idea is that the ocean has qualities that vary as one moves over the watery surface of the planet, such that there are discrete regions of sea.
You usually say the sky. When sky is used with an adjective, use a… sky. You can also use the plural form skies, especially when you are thinking about the great extent of the sky Oxford Advanced Learner's Dictionary 9th edition © Oxford University Press, 2015
It is also an aviation term.
As in the Open Skies Treaty. This has to do with political or administrative clearance, something that as of today, 2020 March 12, is in the news due to travel restrictions related to the COVID-19 virus. (Notice how neutral I stayed on that issue.)
Also as in a pilot being friendly and wishing you clear skies. This refers to both good flying weather clear of clouds, and little to no traffic, meaning clear flying. (The article is about proposed changes in the language pilots use. These changes are intended to keep planes from hitting each other.)
Today the sky over London may be cloudy but the same sky over Mumbai may be clear. If you are in London you will say the sky is cloudy while being in Mumbai I will say it is clear over here. Similarly for a pilot "weather conditions" are different for every 20 kilometres, if he finds a clear weather everywhere, he will inform the authority "the skies are very clear".
Moreover, a poet can describe Egypt as "Egypt is a place of endless blue skies and deserts," just to make you feel the vastness and beauty of the place.
