Learn Hardcore Japanese: fuyu ni samuku narimasu.

If you used 冬は, you’d be making “winter” the topic rather than specifying time.
Other time examples: 朝に起きます (“I wake up in the morning”), 三時に終わります (“It ends at three o’clock”).

Why is bolded 寒い changed to bolded 寒く?

When you attach an adjective to なる (to become), Japanese requires the adjective’s adverbial form.

For i-adjectives, replace the final い with く.
So 寒い → 寒く
- なる = “to become cold.”
Other example: 赤い (red) → 赤くなる (“to become red”).

What role does bolded なる play in this sentence?

bolded なる means “to become.” Combined with an adjective’s adverbial form, it expresses a change of state:

寒くなる: “(It) becomes cold.”
You’re literally saying “In winter, it becomes cold.”

How is bolded 寒くなる different from simply saying bolded 寒いです?

寒いです states a current condition: “It is cold.”
寒くなります emphasizes a change or transition: “It gets cold” or “It becomes cold.”
Use ～くなる when you want to talk about something changing into that state.

Why is there no subject in this sentence?

Japanese often omits subjects when they’re clear from context. Here, the speaker assumes you know they’re talking about the weather or the environment in general.

Full version (explicit subject): 冬には天気が寒くなります.
But native speakers usually drop 天気が (“the weather”) or それが (“it”).

Could we use bolded 冬は instead of bolded 冬に? How would the meaning change?

Yes, but the nuance shifts:

冬は寒くなります treats “winter” as the topic (“As for winter, it gets cold”). It sounds more like a general comment about winter.
冬に寒くなります focuses on when the change happens (“When winter comes, it gets cold”).

Why is the ending bolded なります in the polite form? Can I use the plain form?

なります is the polite present/future form. It’s suitable for conversations with people you don’t know well or in formal contexts.
The plain form is なる: 冬に寒くなる. Use it with friends or in casual writing.

Could I say bolded 冬になると寒くなります? What does the bolded と add?

Yes. 冬になると寒くなります uses the conditional と, meaning “whenever/once winter arrives, it gets cold.”

…と… expresses a natural or automatic consequence.
It’s a bit more natural if you want to emphasize the “once X happens, Y follows” pattern.

How do you pronounce bolded 寒くなります?

It’s pronounced: sa-mu-ku na-ri-ma-su

さ (sa) – む (mu) – く (ku) – な (na) – り (ri) – ま (ma) – す (su)
The pitch accent may vary by dialect, but in standard Tokyo Japanese, it’s Heiban (flat) or with a slight drop after the first mora (sa).