Des que ha començat l'octubre, el cel està més ennuvolat al matí.

Des que means since when it introduces a clause with a verb:

• Des que ha començat l'octubre... = Since October began...

By contrast, des de is usually followed by a noun, time expression, or place, not a full clause:

• des de dilluns = since Monday
• des de casa = from home

So here des que is used because it is followed by the verb phrase ha començat.

Ha començat is the present perfect in Catalan, built with ha + past participle (començat).

In this sentence, it fits well because the idea is:

• October began in the past
• and the situation is still relevant now

That matches the common use of the present perfect after des que when talking about something that started and still matters in the present.

A learner may compare:

• Des que ha començat l'octubre... = Since October started...
• Quan va començar l'octubre... = When October started...

The second one would sound more like a finished past event, not an ongoing since relationship.

Catalan often uses the definite article with months, especially in many normal sentence contexts.

So:

• l'octubre = October

The apostrophe appears because el becomes l' before a vowel:

• el + octubre → l'octubre

This is very common in Catalan:

• l'abril
• l'agost
• l'octubre

English usually does not use the with months, but Catalan often does.

Catalan has two verbs for to be: ser and estar.

Here, està més ennuvolat uses estar because it describes a state or condition of the sky at this time, not a permanent defining quality.

• El cel està ennuvolat = The sky is cloudy / clouded over
• El cel és blau = The sky is blue

A rough guide:

• ser: identity, essential characteristics
• estar: states, conditions, situations

So està més ennuvolat means the sky is currently in a cloudier state.

Ennuvolat means cloudy, cloud-covered, or overcast/clouded over, depending on context.

It comes from the idea of being covered with clouds:

• núvol = cloud
• ennuvolat = clouded over

So:

• el cel està ennuvolat = the sky is cloudy
• més ennuvolat = cloudier / more overcast

Catalan usually forms the comparative with més + adjective:

• més alt = taller
• més interessant = more interesting
• més ennuvolat = cloudier / more cloudy

English sometimes has a special -er form, but Catalan normally just uses més.

Because the comparison is implicit, not fully stated.

• està més ennuvolat = it is cloudier / more overcast
  • compared with before, or compared with usual conditions

If you want to say what it is being compared to, then you add que:

• El cel està més ennuvolat que al setembre. = The sky is cloudier than in September.

So in your sentence, que is not needed because the reference point is understood from context.

Al matí means in the morning.

It is made from:

• a
  • el → al

So literally:

• al matí = at the morning but in natural English we translate it as in the morning.

This contraction is very common:

• al vespre = in the evening
• al carrer = in the street / on the street
• al matí = in the morning

Catalan often puts time expressions at the end of the sentence, especially when they modify the main statement naturally.

So:

• el cel està més ennuvolat al matí

is a very normal order.

It highlights the main statement first:

• the sky is cloudier and then adds
• in the morning

You could move parts around in some contexts, but this version is very natural and neutral.

That opening clause sets the time frame first.

It is similar to English:

• Since October started, ...

Putting it first helps the listener understand from the beginning that the statement is about a change that has been true since a certain point in time.

The comma is also natural here because the sentence starts with a subordinate clause and then moves to the main clause.

Yes. El cel is the normal Catalan expression for the sky.

• cel = sky / heaven, depending on context
• el cel = the sky

In a weather sentence, el cel is exactly what you would expect:

• El cel està clar = The sky is clear
• El cel està ennuvolat = The sky is cloudy

Yes. Començat is the past participle of començar (to begin / to start).

So:

• començar = infinitive
• ha començat = has begun / started

This is a very common pattern in Catalan:

• ha arribat from arribar
• ha canviat from canviar
• ha començat from començar

Not in the same structure.

You cannot say:

• Des de ha començat l'octubre ❌

because des de is not used before a finite verb clause.

You would need a noun phrase instead, for example:

• Des de començaments d'octubre, el cel està més ennuvolat al matí. = Since the beginning of October, the sky is cloudier in the morning.

So:

• des que + verb
• des de + noun/time expression

Més means more.

It forms the comparative with the adjective:

• ennuvolat = cloudy
• més ennuvolat = more cloudy / cloudier

So the structure is:

• està + més + adjective

Examples:

• està més fred = it is colder
• està més fosc = it is darker
• està més ennuvolat = it is cloudier