Em janeiro, o clima nesta cidade é estranho.

to be

Breakdown of Em janeiro, o clima nesta cidade é estranho.

• esta: this
• a cidade: the city
• em: in

Questions & Answers about Em janeiro, o clima nesta cidade é estranho.

Why is it em janeiro and not no janeiro?

In Portuguese, names of months normally do not take the definite article, so you say em janeiro (in January), em fevereiro, etc.
No is a contraction of em + o (in + the), so no janeiro would literally be “in the January”, which sounds wrong in this sentence.

You do see the article if there’s a word like mês:

• no mês de janeiro = in the month of January

Is the comma after Em janeiro necessary?

When a short time expression like Em janeiro comes at the start of the sentence, a comma is common and considered correct: it separates the time frame from the main clause.
You will sometimes see it written without a comma in informal texts, but in careful writing Em janeiro, o clima… is preferred.

Why isn’t janeiro capitalized like January in English?

In Portuguese, names of months and days of the week are not capitalized, unless they begin a sentence or are in all caps for design reasons.
So we write janeiro, fevereiro, março (January, February, March).
In your sentence, Em is capitalized because it starts the sentence, but janeiro stays lower‑case.