What's the difference between になり and になる?

Question

Came across this sentence while reading:

もう雲は鼠色になり、向うの山には山焼けの火がまっ赤です。

Assuming I'm reading it right, I'd expect it to use なる, not its masu stem.
e.g.

もう雲は鼠色になる、向うの山には山焼けの火がまっ赤です。

Is になり just another way of saying "is" or "be" as in "Already the clouds are grey"? Or is there some greater nuance?

EDIT:
Chocolate posted some great links in the comments on this post explaining how a verb's stem form can be used as a conjunction.
Is there a term for using conjugating verbs such that the sentence continues with another clause?
and なく vs. なくて and stem form vs. てform as conjunctions

Answer 1

It is entirely different. なり here is the conjunctive form of the verb なる (become). You can rewrite it as なって if that's clearer for you.

もう雲は鼠色になり、向うの山には山焼けの火がまっ赤です。

and

もう雲は鼠色になって、向うの山には山焼けの火がまっ赤です。

means the same (The clouds turned grey and the sun, setting on the mountain in front, is crimson), but the former is more formal.

On the other hand, if なる appears in the middle of a sentence that means that we are dealing with the attributive form of なる. (However, since there is a comma なる feels a bit unexpected and the meaning of the sentence is unusual.)

もう雲は [鼠色になる向うの山] には山焼けの火がまっ赤です。

As for the clouds, on the mountain which will turn grey, the sunset is crimson.

Which is more or less gibberish.

I know that my translations are far from perfect but they should be enough to point out the big difference between なり and なる here. In case you want to improve the translations please feel free to do so.