Quan hi ha núvols sobre la muntanya, no vull sortir sense paraigua.

Hi ha is the very common Catalan way to say there is or there are.

So:

• hi ha un núvol = there is a cloud
• hi ha núvols = there are clouds

In your sentence, Quan hi ha núvols... means When/Whenever there are clouds...

The little word hi does not translate neatly by itself here; it is just part of the fixed expression haver-hi for existence.

In standard Catalan, existential haver-hi normally stays in the singular form:

• hi ha un llibre
• hi ha dos llibres

So even with a plural noun like núvols, you still usually say hi ha.

This is different from English, where we say there is / there are.

It can feel like whenever in this kind of sentence.

Quan literally means when, but in general statements or habits it often has the sense of whenever:

• Quan plou, em quedo a casa = When/Whenever it rains, I stay home.

So in your sentence, Quan hi ha núvols sobre la muntanya... suggests a repeated or usual situation, not just one specific moment.

Yes, but the nuance changes a little.

• Quan hi ha núvols... = when / whenever there are clouds
• Si hi ha núvols... = if there are clouds

Quan sounds more like something expected or regularly observed.
Si sounds more conditional: it may or may not happen.

In many everyday contexts, both could work, but quan fits well if the speaker is talking about a typical pattern.

In Catalan, no normally goes directly before the conjugated verb:

• No menjo
• No puc venir
• No vull sortir

Since vull is the conjugated verb and sortir is an infinitive, no goes before vull, not before sortir.

So:

• no vull sortir = I do not want to go out

This is a very common Catalan pattern:

conjugated verb + infinitive

Here:

• vull = I want
• sortir = to go out / to leave

So vull sortir literally means I want to go out.

Other examples:

• vull menjar = I want to eat
• vull dormir = I want to sleep

Sortir can mean both to leave and to go out, depending on context.

In this sentence, sortir is best understood as to go out, because the sentence is about weather and taking an umbrella.

Common uses:

• surto de casa = I leave the house
• vull sortir = I want to go out
• sortim aquesta nit = we’re going out tonight

After sense (without), Catalan often uses a noun without an article when the meaning is general:

• sense diners = without money
• sense temps = without time
• sense paraigua = without an umbrella

This sounds natural when you mean without any umbrella / without carrying an umbrella.

You can sometimes hear sense un paraigua, but sense paraigua is very idiomatic and common.

Sobre usually means over, above, or on/about depending on context.

Here, with weather and clouds, sobre la muntanya means over the mountain or above the mountain.

So it describes where the clouds are located.

If you said a la muntanya, that would mean on the mountain / in the mountains, which is a different idea.

Because the two nouns are being used differently.

• la muntanya refers to a specific mountain, or at least a definite one in the context.
• paraigua after sense is indefinite and general, so no article is needed.

Compare:

• sobre la muntanya = over the mountain
• sense paraigua = without an umbrella

So the article depends on whether the noun is definite, indefinite, or used generically.

Vull is usually pronounced roughly like booly without a full final y, or more carefully vuy / bui depending on accent and dialect.

A few useful points:

• the v is often pronounced like b in most Catalan accents
• ull here does not sound like English ull
• it is a short one-syllable word

If you are aiming for a central Catalan pronunciation, vull sounds close to bool with a palatalized ending. The exact sound takes practice, but the important thing is that it does not sound like English vull.

• núvols is stressed on the first syllable: NÚ-vols
• paraigua is stressed on AI: pa-RAI-gua

The written accent in núvols tells you the stress clearly.

Stress matters in Catalan, so it is worth learning words together with their stressed syllable.

Yes. Catalan often uses the present tense for general truths, habits, and repeated situations.

So:

• Quan hi ha núvols... = When/Whenever there are clouds...
• no vull sortir... = I do not want to go out...

This is completely normal and works much like the English present in sentences such as When it rains, I stay home.