Al setembre i a l'octubre, el matí és més humit que a l'agost.

Both mean in a month, but the forms are built differently.

• a + el = al
• Before a vowel, el becomes l'

So:

• a + el setembre → al setembre
• a + l'octubre stays a l'octubre

A natural way to think of it is: Catalan often uses a + definite article with months in time expressions like this.

Because the masculine singular article el is shortened to l' before a vowel sound.

So:

• el setembre
• l'octubre
• l'agost

This is very common in Catalan:

• el llibre
• l'amic
• l'hotel

Catalan normally uses the definite article with parts of the day:

• el matí = the morning
• la tarda = the afternoon
• la nit = the night

So el matí és més humit... sounds natural in Catalan. Leaving out the article would usually sound incomplete or unnatural here.

In Catalan, the singular can refer to the time period in general.

So el matí és més humit means something like:

• the morning is more humid
• or more naturally in English, mornings are more humid

If you said els matins, that would sound a bit more like repeated or habitual mornings. The singular el matí is a normal general statement about that part of the day.

Because this sentence gives a general characteristic or comparison, not a temporary state of one specific morning.

• ser (és) is used for general descriptions, classifications, and comparisons
• estar is more for states or conditions in a particular situation

Here, the idea is that mornings in those months are generally more humid than in August, so és is the natural choice.

It is the standard comparative pattern:

• més = more
• adjective = humit
• que = than

So:

• més humit que = more humid than

This is exactly the same structure you see in many Catalan comparisons:

• més gran que = bigger than
• més fàcil que = easier than
• més car que = more expensive than

Because a l'agost means in August.

The comparison is really between time periods:

• in September and October
• versus in August

So que a l'agost means than in August.

Even though English uses just than August only in some shortened phrases, Catalan keeps the time preposition here.

Because adjectives agree with the noun they describe.

Here, the noun is el matí, which is:

• masculine
• singular

So the adjective must also be masculine singular:

• humit

Other forms would be:

• humida = feminine singular
• humits = masculine plural
• humides = feminine plural

They help with both pronunciation and meaning.

• és = is
  This distinguishes it from es, which is a pronoun.
• més = more
  This distinguishes it from mes, which means month.
• matí has the accent to show the stress falls on the last syllable: ma-TÍ

So the accents are important and should not be ignored.

Yes. The sentence starts with the time phrase for emphasis or framing:

• Al setembre i a l'octubre, el matí és més humit que a l'agost.

You could also say something like:

• El matí és més humit al setembre i a l'octubre que a l'agost.

That is still understandable, but the original order is very natural because it sets the time context first.

It is natural because the sentence begins with a fronted time expression:

• Al setembre i a l'octubre, ...

The comma helps separate the setting from the main statement. In some short sentences, people may omit such a comma, but using it here is clear and standard.