"It gets colder and colder in the winter."
"Become + adjective" in English is "--ku naru" in Japanese (--ku being the conjunctive ending of the Japanese adjective).
Not to confuse anyone, but this might be a good time to point out that there is a big difference between English adjectives and Japanese "keiyoodooshi." The Japanese variety are fully conjugated predicates, with verbal syntax and grammar. "Samuku naru" is more of a compound verb than an adverb-verb combination.