Com més humit és l'aire, menys vull sortir a córrer al matí.

It is the Catalan pattern for the more ..., the less ....

So:

• Com més humit és l'aire = The more humid the air is
• menys vull sortir a córrer = the less I want to go running

This structure shows a relationship between two changes:

• com més ..., més ... = the more ..., the more ...
• com més ..., menys ... = the more ..., the less ...
• com menys ..., més ... = the less ..., the more ...

For example:

• Com més estudio, millor parlo. = The more I study, the better I speak.
• Com menys dormo, més cansat estic. = The less I sleep, the more tired I am.

Yes, com often means how or like/as, but here it is part of a fixed comparative structure: com més / com menys.

In this pattern, com does not translate word-for-word by itself. The whole expression works together:

• com més = the more
• com menys = the less

A learner often wants to use quan or quant, but those are different:

• quan = when
• quant = how much / how many

So in this sentence, com is simply the correct word for this structure.

Because humit agrees with l'aire, and aire is a masculine singular noun.

So:

• l'aire = masculine singular
• therefore the adjective must be humit = masculine singular

Compare:

• l'aire humit = the humid air
• la roba humida = the damp clothing

Even though l'aire starts with l', it is still masculine. The apostrophe does not tell you the gender by itself.

Because the article is shortened before a vowel.

The full masculine article is el, but before a vowel sound it becomes l':

• el noi = the boy
• l'home = the man
• l'aire = the air

So l'aire is just el aire contracted the way Catalan normally does it.

In this sentence, és humit is the natural way to describe the air as humid.

Very roughly:

• ser often describes qualities or characteristics
• estar often describes states or conditions

With something like air, Catalan commonly uses ser humit to talk about humidity in the atmosphere. By contrast, estar humit is very common for things that are physically damp or wet:

• La tovallola està humida. = The towel is damp.
• L'aire és humit. = The air is humid.

Also, the verb is needed here. Standard Catalan wants the full clause com més humit és l'aire.

Because Catalan often leaves out subject pronouns when the verb already makes the subject clear.

Here, vull means I want, so jo is not necessary.

• vull = I want
• vols = you want
• vol = he/she/it wants

So menys vull sortir... already clearly means the less I want to go out...

You can add jo for emphasis or contrast:

• Jo vull sortir, però ell no. = I want to go out, but he doesn’t.

Because in this correlative structure, the comparative word is normally placed first for emphasis and parallelism:

• Com més ..., menys ...

So the sentence mirrors itself:

• Com més humit...
• menys vull...

Also, changing the position changes the meaning or makes the sentence sound unnatural:

• menys vull sortir a córrer = I want it less
• vull sortir menys a córrer = I want to go running less often / less much
• vull menys sortir = not natural here

So menys is modifying the degree of wanting, not the frequency of going out.

Sortir a córrer means to go out for a run or to go running.

It suggests leaving where you are in order to run.

• sortir = to go out
• córrer = to run

So literally it is something like go out to run.

Anar a córrer is also very common and often very close in meaning:

• sortir a córrer = go out running / go out for a run
• anar a córrer = go running

The difference is subtle:

• sortir emphasizes going out
• anar is a more neutral go

Because after verbs of movement like sortir, Catalan often uses a + infinitive to express purpose.

So:

• sortir a córrer = go out to run
• anar a comprar = go to buy / go shopping
• venir a veure = come to see

Here, the a belongs with sortir, not with vull.

The structure is:

• vull = I want
• sortir a córrer = to go out running

So the whole thing means I want to go out running.

Al is the contraction of a + el.

• a + el = al

So:

• al matí = in the morning

This is a very common Catalan time expression. Similar phrases are:

• al matí = in the morning
• a la tarda = in the afternoon
• al vespre = in the evening
• a la nit = at night

Depending on context, al matí can mean either:

• in the morning (one specific morning)
• in the mornings (as a habit)

Out of context, it most naturally sounds like a general tendency or habit:

The more humid the air is, the less I want to go running in the morning.

That is, whenever the air is more humid, your desire to run goes down.

Catalan often uses the present tense this way, just like English:

• Quan plou, em quedo a casa. = When it rains, I stay home.
• Com més humit és l'aire, menys vull sortir... = The more humid the air is, the less I want to go out...

Context could make it refer to a current situation, but by itself it sounds general.

A few useful notes:

• humit: the h is silent
• menys: ny is a single sound, like the ny in canyon
• córrer: the rr is a strong/rolled r
• matí: stress is on the last syllable

Very rough pronunciation guides:

• humit ≈ oo-MEET
• més ≈ mess
• és ≈ ess
• córrer ≈ COR-rer
• matí ≈ ma-TEE

About the accent marks:

• més and és have accents to mark the stressed vowel and to distinguish them from other words
• córrer has ò, which also tells you the vowel quality
• matí has an accent because the stress falls on the final syllable

So the accents are not optional spelling details; they are part of the word.