A l'agost, l'aire és més sec i calent, però al setembre el matí ja és una mica més humit.

Both mean in a month, and both use the preposition a plus the definite article.

• a + el setembre → al setembre
• a + l'agost because agost begins with a vowel, so el becomes l'

So:

• a l'agost = in August
• al setembre = in September

This is a very common pattern in Catalan.

In Catalan, the names of months are normally written with lowercase, not capitals.

So you write:

• agost
• setembre

This is different from English, where August and September are capitalized.

This is because the article el is shortened before a vowel.

• el agost → l'agost
• el aire → l'aire

The apostrophe shows that the vowel in el has dropped.

This is very common in Catalan:

• l'home
• l'escola
• l'aire

Catalan can use més once and let it apply to both adjectives.

So més sec i calent means:

• drier and hotter

You could repeat més for extra clarity or emphasis:

• més sec i més calent

But in a sentence like this, using més only once sounds natural.

Because they agree with the nouns they describe:

• l'aire is masculine singular
• el matí is masculine singular

So the adjectives are also masculine singular:

• sec
• calent
• humit

If the noun were feminine, the adjective would usually change:

• l'aire → sec
• la tarda → seca (if that adjective made sense in context)

Agreement is an important feature of Catalan grammar.

Here ja means something like already, by then, or now already.

In al setembre el matí ja és una mica més humit, it suggests that by September, a change has started to be noticeable.

So ja adds the idea:

• by September, the morning is already a bit more humid

It often marks a transition or a new stage.

Una mica means a little or a bit.

So:

• una mica més humit = a little more humid
• slightly more humid

It softens the comparison. Without it:

• més humit = more humid

With it:

• una mica més humit = a bit more humid

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

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

In this sentence, el matí is the subject, so the article is natural.

Catalan uses articles more often than English in cases like this.

The time expressions are placed first to set the scene:

• A l'agost = in August
• al setembre = in September

This is very natural in Catalan, especially when contrasting two time periods.

The sentence is basically organized like this:

• In August, the air...
• but in September, the morning...

Putting the time phrase first helps highlight the contrast between the two months.

Però means but.

It introduces a contrast between the first and second parts of the sentence:

• August: the air is drier and hotter
• September: the morning is already a bit more humid

So però connects two contrasting ideas, just like but in English.

The h in Catalan is normally silent, so humit does not begin with an English h sound.

It is pronounced roughly like oo-MEET.

Also note:

• the stress falls on the second syllable: hu-MIT

This silent h is normal in Catalan:

• home
• hora
• humit

Here calent means hot.

So més sec i calent means:

• drier and hotter

In weather and climate descriptions, calent is the normal everyday word for hot. It is a very common adjective.

Not perfectly. The grammar is natural in Catalan, but English would usually phrase it a bit differently.

For example:

• A l'agost, l'aire és més sec i calent
  literally: In August, the air is drier and hot/warm
• però al setembre el matí ja és una mica més humit
  literally: but in September the morning is already a little more humid

English usually prefers smoother phrasing, but the Catalan structure itself is completely normal.