Está frio, especialmente à noite.

Because Portuguese usually uses estar for weather and temporary conditions.

• Está frio = It’s cold
• Está quente = It’s hot
• Está nublado = It’s cloudy

Using ser here would sound unnatural in normal speech. In general:

• ser = essential, defining characteristics
• estar = current state or condition

Weather is normally treated as a condition, so está frio is the natural choice.

Portuguese often leaves out the subject when it is understood from the verb or when English uses a “dummy” subject like it.

In English, you say:

• It’s cold
• It’s raining

But in Portuguese, you normally just say:

• Está frio
• Está chovendo

That means Portuguese does not need an equivalent of English weather it in this kind of sentence.

Here, frio means cold and works like an adjective describing the current condition.

So:

• Está frio = It’s cold

You will also see frio as a noun in other contexts:

• O frio chegou. = The cold has arrived.

But in your sentence, it is best understood as an adjective after estar.

Especialmente means especially, and here it adds extra information about when the cold is more noticeable.

The sentence structure is:

• Está frio = main statement
• especialmente à noite = extra detail: especially at night

This placement is very natural in Portuguese. You could think of it as:

• It’s cold, especially at night.

Because à is a contraction of:

• a (preposition: at/to)
• a (the feminine article the)

So:

• a + a = à

In à noite, the idea is basically at the night / at night, and Portuguese uses the contracted form à.

This accent mark is called a crase.

Compare:

• à noite = at night
• à tarde = in the afternoon
• à direita = to the right / on the right

Yes. In Brazilian Portuguese, à noite and de noite can both mean at night.

However, there is a slight difference in feel:

• à noite is often seen as a bit more neutral or standard in many contexts.
• de noite is very common in everyday Brazilian speech.

Examples:

• Está frio, especialmente à noite.
• Está frio, especialmente de noite.

Both are correct in Brazil.

No. Another very common option is:

• Faz frio. = It’s cold.

Both are natural, but they can feel slightly different:

• Está frio focuses on the current condition.
• Faz frio is also very common for weather in general.

Examples:

• Hoje está frio. = Today it’s cold.
• No inverno faz frio aqui. = In winter it’s cold here.

In your sentence, está frio sounds completely natural.

A simple approximate pronunciation for an English speaker is:

• es-TAH FREE-oo, es-peh-see-al-MEN-chee ah NOY-chee

A few helpful notes for Brazilian Portuguese:

• está has stress on the last syllable: -tá
• frio usually sounds like FREE-oo
• especialmente in Brazil often has a chee sound at the end: -mente
• noite sounds roughly like NOY-chee in many Brazilian accents

Pronunciation varies by region, but this will get you close.

The comma separates the main statement from the extra comment.

• Está frio = main idea
• especialmente à noite = additional information

This is very similar to English:

• It’s cold, especially at night.

The comma makes the sentence easier to read and sounds natural in writing.

Yes, but the meaning changes slightly.

• Está frio à noite. = It’s cold at night.
• Está frio, especialmente à noite. = It’s cold, especially at night.

With especialmente, you are saying that it is cold in general, but nighttime is when this is especially true.

So especialmente adds emphasis.

Yes, it is completely natural.

A Brazilian speaker would easily say or understand:

• Está frio, especialmente à noite.

It sounds like normal everyday Portuguese and is a good model sentence for talking about weather.