Après la pluie, on voit un arc-en-ciel au-dessus de la mer.

In French, on is very often used where English might use we, you, or people in general.

Here, on voit does not necessarily mean that a specific group is seeing the rainbow. It often means something like:

• we can see
• you can see
• one sees
• people see

It is a very natural, everyday way to express a general observation.

Because on is grammatically treated as third person singular, even when it means we in everyday French.

So:

• on voit = singular verb form
• nous voyons = first person plural

Even if on can mean we, the verb still stays singular.

These two expressions are related, but they are not used in exactly the same way.

• il y a un arc-en-ciel = there is a rainbow
• on voit un arc-en-ciel = you/we can see a rainbow

So il y a simply states existence, while on voit focuses on perception or what is visible.

In this sentence, on voit sounds natural because it describes what appears to the eye after the rain.

In French, many general ideas that English expresses without an article use the definite article.

So French says:

• après la pluie = literally after the rain

This is the normal way to talk about the weather event rain in a general sense here.
Après pluie would sound incomplete or unnatural in standard French.

Après la pluie is a time expression placed at the beginning of the sentence.

The comma helps separate that introductory phrase from the main clause:

• Après la pluie, = after the rain
• on voit un arc-en-ciel... = one sees / you can see a rainbow...

In French, this comma is common and helps readability, though punctuation can sometimes vary.

Yes, arc-en-ciel is a masculine noun, which is why it uses un.

So:

• un arc-en-ciel = a rainbow

This is just something you need to learn with the noun: arc-en-ciel is masculine.

Because arc-en-ciel is a fixed compound noun in French.

It is made from several elements:

• arc = arc/bow
• en = in
• ciel = sky

Together, they form the standard word for rainbow, and French writes it with hyphens: arc-en-ciel.

The usual modern plural is:

• des arcs-en-ciel

Only arc normally takes the plural s.

So:

• un arc-en-ciel
• des arcs-en-ciel

You may sometimes encounter variation in older sources, but arcs-en-ciel is the standard form to learn.

Au-dessus de means above or over, while sur usually means on or on top of.

So:

• au-dessus de la mer = above the sea
• sur la mer = on the sea / on top of the sea

A rainbow is not physically resting on the sea, so au-dessus de is the correct choice.

Because au-dessus de is followed by de, and then the noun keeps its normal article if needed.

Here the noun is la mer, so:

• au-dessus de la mer

This is not the same as the article contraction in forms like à + le = au or de + le = du.
Since mer is feminine singular, you get de la mer.

Compare:

• au-dessus du jardin = above the garden
• au-dessus de la mer = above the sea

In normal pronunciation:

• on sounds roughly like a nasal ohn
• voit sounds like vwa

The final t in voit is not pronounced.

So on voit is pronounced approximately on vwa.

Also, there is no special liaison between on and voit here.

It is pronounced approximately:

• ark-an-syel

A few helpful points:

• the c in arc is pronounced
• en has a nasal vowel
• ciel sounds like syel

Even though it has hyphens in writing, it is spoken smoothly as one unit.

Yes, you could say:

• Après la pluie, nous voyons un arc-en-ciel au-dessus de la mer.

But this sounds more specifically like we see a rainbow, referring to a particular group including the speaker.

By contrast, on voit is more natural for a general statement or casual spoken French. That is why on voit is often preferred in everyday language.

Yes. French commonly puts a time phrase first for emphasis or context:

• Après la pluie,
  • main statement

Then the rest follows in a normal order:

• subject: on
• verb: voit
• object: un arc-en-ciel
• place phrase: au-dessus de la mer

So the sentence structure is very natural and typical:

time expression + subject + verb + object + place