Learn Hardcore Portuguese (Portugal): Em janeiro, o clima nesta cidade é estranho. - In January, the climate in this city is strange.

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.

What exactly does nesta mean, and why not em esta cidade?

Nesta is a contraction of em + esta:

em = in
esta = this (feminine, singular)
So nesta cidade literally means “in this city”.

In standard Portuguese you must contract em + esta → nesta.
Saying em esta cidade sounds foreign or very unnatural in modern European Portuguese.

Why is it nesta cidade (feminine) and not neste cidade?

Demonstratives must agree with the noun in gender and number:

cidade is feminine singular, so you use esta / nesta.
neste is masculine singular and would be used with a masculine noun: neste país (in this country).

Patterns:

masculine: neste (em + este)
feminine: nesta (em + esta)
plural masculine: nestes
plural feminine: nestas

Why use clima instead of tempo for talking about the weather?

In Portuguese:

o clima usually refers to climate, the general long‑term pattern, or sometimes a figurative “atmosphere” (e.g. in a room, in a country).
o tempo is the everyday word for weather (as well as “time”).

In European Portuguese, people very often say:

Em janeiro, o tempo nesta cidade está estranho. (In January, the weather in this city is strange.)

Your sentence with clima is still perfectly correct; it sounds a bit more like you’re commenting on the typical January climate there, not just today’s weather.

Why is there a definite article: o clima? Could I say just clima nesta cidade é estranho?

In Portuguese, general nouns like clima, tempo, comida, música almost always take a definite article when used as subjects.
So O clima nesta cidade é estranho is normal and natural.

Saying Clima nesta cidade é estranho (without o) sounds incomplete or foreign, a bit like saying “Climate in this city is strange” in English.

Why do we say é estranho and not está estranho?

The choice between ser (é) and estar (está) shows how you see the situation:

é estranho (with ser) suggests a more general or characteristic fact: “As a rule, in January the climate of this city is strange.”
está estranho (with estar) focuses on a temporary or current state: “Right now, it’s strange.”

So your sentence presents the “strangeness” as a kind of typical feature of January in that city.
If you were talking about this particular January only, you could say:

Este janeiro, o clima nesta cidade está estranho.

Could I move estranho and say o clima estranho nesta cidade or o estranho clima nesta cidade?

Yes, but the meaning and emphasis shift slightly:

O clima nesta cidade é estranho.
- Neutral, just states a fact: “The climate is strange.”
O clima estranho desta/nesta cidade…
- More descriptive, often used when the climate is already a known topic: “the strange climate of this city.”
O estranho clima desta/nesta cidade…
- Puts extra emphasis on how strange it is; more stylistic/literary.

In everyday speech, your original structure (…é estranho) is the most common and straightforward.

Why is cidade feminine? Are nouns ending in -dade usually feminine?

Yes. In Portuguese, almost all abstract nouns ending in -dade are feminine:

a cidade (city)
a liberdade (freedom)
a amizade (friendship)
a vontade (will, desire)

So you say a cidade, esta cidade, nesta cidade, and adjectives agree in the feminine if they need to (e.g. cidade grande, not cidade grandeo).

How do you pronounce estranho in European Portuguese, and what sound does nh make?

In European Portuguese, estranho is roughly like “sh-TRUN-yoo”:

es- often sounds like “sh” before t: something like “esh-” or “sh”.
-tra- = a quick tr with a tapped r.
-nh- is a single consonant, like the “ny” in English “canyon” or “lasagna”.
final -o is usually a short “oo” sound (u) in European Portuguese.

So nh always represents that “ny” sound:

banho (bath) ≈ BA-nyoo
vinho (wine) ≈ VEE-nyoo

Is the m at the end of em actually pronounced?

The final m in em isn’t pronounced as a full [m].
Instead, it nasalizes the vowel, so em is a nasal “eh” / “ey” sound, often close to something like “ẽ” or “ẽy”.

In fast European Portuguese speech, Em janeiro can sound almost like “ẽ janeiro” or “ẽy janeiro”, with no clear separate m.

Can I say nesta cidade aqui or is that redundant?

You can say nesta cidade aqui in spoken Portuguese for extra emphasis, and people do it.

nesta cidade already means “in this city”.
adding aqui (here) makes it more emphatic or contrastive: “in this city here (as opposed to elsewhere).”

It’s not wrong, just a bit more informal and emphatic than plain nesta cidade.