Cuando la nube pasa, el cielo se vuelve azul otra vez.

Because this sentence describes a general truth or habitual action (“whenever a cloud passes…”). Spanish uses the present indicative for routine or real events. If you were referring to a specific future occurrence, you’d use the subjunctive with a future tense in the main clause:

• Cuando pase la nube, el cielo se volverá azul otra vez.
(“Once the cloud passes, the sky will turn blue again.”)

Volverse (with se) means “to become” or “to turn into,” emphasizing a change of state. The se marks it as a pronominal verb indicating that the subject (el cielo) experiences this change:

• El cielo se vuelve azul = “The sky becomes blue.”

Yes, ponerse + adjective is also common for sudden or visible changes (especially colors or moods):

• Cuando la nube pasa, el cielo se pone azul otra vez.

Differences:

• Ponerse often implies a more temporary or reversible change.
• Volverse can suggest a more noticeable or fundamental shift.
  In everyday speech either choice is fine for colors.

Spanish generally uses the definite article with singular, countable nouns when talking about them in a general or specific sense. Here you’re referring to “the cloud” that’s passing by and “the sky” overhead. Omitting the articles would sound unnatural:

• Cuando nube pasa… ❌
• Cuando la nube pasa… ✅

No. Spanish requires the articles in this context to make the sentence grammatically correct and natural. Dropping them makes it sound like a word-for-word, ungrammatical translation from English.

Otra vez (“again”) is an adverbial phrase modifying the entire clause se vuelve azul. Spanish typically places such adverbials after the verb or at the end for emphasis:

• El cielo se vuelve azul otra vez.

You could also say otra vez el cielo se vuelve azul, but that shifts the focus.

Yes, a comma separates the subordinate temporal clause (Cuando la nube pasa) from the main clause (el cielo se vuelve azul otra vez). It clarifies the sentence structure:

• Cuando la nube pasa, el cielo se vuelve azul otra vez.

Here pasar is used intransitively to mean “to pass by/through” without an object. You only need por if you explicitly state “to pass by something”:

• La nube pasa (pasar intransitive)
• La nube pasa por la montaña (passes by the mountain)

Yes. Spanish allows both:

• Cuando la nube pasa, …
• Cuando pasa la nube, …

Both are correct; the first emphasizes la nube, the second is slightly more neutral.