Vejret er varmt nok til at sidde på terrassen.

Because vejret is the definite form of vejr.

• et vejr = a weather / weather as a noun
• vejret = the weather

In Danish, when talking about the weather in a general real-world sense, Danish very often uses the definite form:

• Vejret er godt i dag. = The weather is good today.
• Vejret er koldt. = The weather is cold.

So vejret is the normal choice here.

Because vejr is a neuter noun: et vejr.

In Danish, adjectives often agree with the noun:

• en varm dag = a warm day
• et varmt vejr = warm weather

That same neuter -t form is also used when the adjective comes after er:

• Vejret er varmt. = The weather is warm.

So varmt matches vejr / vejret.

Here, nok means enough.

So:

• varmt nok = warm enough

This is a very common pattern in Danish:

• stor nok = big enough
• gammel nok = old enough
• hurtigt nok = fast enough

So Vejret er varmt nok ... means The weather is warm enough ...

This is a standard Danish pattern:

adjective + nok + til at + infinitive

It corresponds to English adjective + enough + to + verb.

Examples:

• Det er varmt nok til at bade. = It is warm enough to swim.
• Hun er gammel nok til at køre bil. = She is old enough to drive a car.
• Vejret er varmt nok til at sidde på terrassen. = The weather is warm enough to sit on the terrace.

So til at sidde is not random: it belongs to the fixed enough to do something pattern.

No, not in this structure.

After nok in this meaning, Danish normally uses:

• nok til at + infinitive

So:

• varmt nok til at sidde = correct
• varmt nok at sidde = not correct in standard Danish

The til is required in this construction.

The Danish verb is at sidde = to sit.

Its forms are:

• infinitive: at sidde
• present: sidder
• past: sad
• past participle: siddet

So in til at sidde, you are seeing the infinitive form, because it comes after at.

På terrassen means on the terrace or more naturally in English, often out on the terrace / patio.

Danish often uses på with places like this:

• på terrassen = on the terrace
• på altanen = on the balcony
• på stranden = on the beach

So even if English might sometimes prefer in or out on, Danish naturally uses på here.

Terrassen is the definite form of terrasse:

• en terrasse = a terrace
• terrassen = the terrace

The ending -en is the Danish way of attaching the to a common-gender noun.

So:

• på terrassen = on the terrace

This usually means a specific terrace, such as the one at the house, restaurant, or place being talked about.

Often, yes, depending on context.

Terrasse can correspond to:

• terrace
• patio
• sometimes outdoor seating area

The exact English translation depends on the situation. But in Danish, på terrassen is a very normal phrase for sitting outside on a terrace or patio area.

Yes, absolutely.

Both are natural, but they emphasize slightly different things:

• Vejret er varmt nok ... = focuses on the weather
• Det er varmt nok ... = focuses on the situation / it is warm enough

Both are idiomatic Danish.

Yes. Nok can also mean probably in other contexts.

Compare:

• Vejret er varmt nok til at sidde på terrassen.
  = The weather is warm enough to sit on the terrace.

• Vejret er nok varmt.
  = The weather is probably warm.

So the position and structure matter a lot:

• adjective + nok + til at ... = enough to ...
• nok earlier in the sentence can often mean probably

This is a very common thing for learners to notice.

The sentence follows normal Danish main-clause word order:

Vejret + er + varmt nok + til at sidde på terrassen

So:

• Vejret = subject
• er = verb
• varmt nok = complement
• til at sidde på terrassen = infinitive phrase explaining what it is warm enough for

This is very similar to English word order in this sentence, which makes it relatively easy to understand.

Yes. Danish is a V2 language, which means the finite verb normally comes in the second position in main clauses.

For example:

• Vejret er varmt nok til at sidde på terrassen.
• I dag er vejret varmt nok til at sidde på terrassen.

When I dag is moved to the front, er still stays in second position, and the subject vejret comes after it.

That is very typical Danish word order.

Yes, it is very natural.

A Danish speaker might say this when deciding whether it is pleasant enough to sit outside. It sounds normal and idiomatic.

Very similar natural alternatives would be:

• Det er varmt nok til at sidde ude.
• Det er varmt nok til at sidde på terrassen.
• Vejret er godt nok til at sidde på terrassen.

But your original sentence is perfectly normal.

Yes, often it does.

Literally, it means warm enough, but in context it usually means comfortably warm enough for that activity.

So it does not necessarily mean the weather is hot. It just means it is warm enough that sitting on the terrace would be pleasant or reasonable.

A rough pronunciation guide might be:

VY-lreth er VARMT nok til at SID-uh po ter-RAS-sen

A few notes:

• vejret is often pronounced more like vaj-ret / vy-ret than the spelling suggests
• varmt has a consonant cluster that may feel a bit tight for English speakers
• på has an aw/o-like vowel that can be tricky
• terrassen has stress on the second syllable: ter-RAS-sen

This is only an approximation, but it may help you recognize the sentence when spoken.