The 連用形（れんようけい）is formed with godan verbs by replacing the -u with -i (so なる becomes なり) and ichidan by removing -ru (食べる becomes 食べ). Like the て form it’s used to link clauses similar to ‘and’, though it’s more formal.
‘The mornings have become cooler, and…’
なり could also be a 終止形 (end-of-sentence form) if we’re talking old language (still used sometimes in literature, proverbs, etc.)
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It’s なる (to become; get)
涼しくなる = to get cooler
The sentence probably does not end there
The 連用形（れんようけい）is formed with godan verbs by replacing the -u with -i (so なる becomes なり) and ichidan by removing -ru (食べる becomes 食べ). Like the て form it’s used to link clauses similar to ‘and’, though it’s more formal.
‘The mornings have become cooler, and…’
なり could also be a 終止形 (end-of-sentence form) if we’re talking old language (still used sometimes in literature, proverbs, etc.)