A medida que caía la tarde, el mar se volvía más frío y el aire más agradable.

A medida que means as or as/while, with the idea that one change happens progressively alongside another.

So in this sentence, it means something like:

As the evening was falling, the sea was becoming colder and the air more pleasant.

It is often used for gradual parallel change:

• A medida que pasa el tiempo, aprendo más.
  As time goes by, I learn more.

It is more specific than just cuando (when), because a medida que emphasizes a process unfolding over time.

Caía is the imperfect, and it is used because the sentence describes a background process in progress, not a single completed event.

Here, caía la tarde means something like:

• the evening was falling
• it was getting later
• night was approaching

The imperfect fits well because the sentence paints a scene and shows things changing gradually.

Compare:

• A medida que caía la tarde... = As evening was falling...
  ongoing, descriptive
• La tarde cayó. = The evening fell.
  completed event, much less natural in this kind of description

So caía is used because the speaker is describing an unfolding atmosphere.

Literally, caía la tarde means the afternoon/evening was falling.

This is an idiomatic and quite natural literary-style expression in Spanish. It refers to the time when the day is ending and evening is setting in.

Depending on context, you could think of it as:

• evening was coming on
• night was drawing in
• the day was fading

It is not about something physically falling. It is a set expression used to describe the transition toward evening.

Very similar expressions include:

• caía la noche = night was falling
• al caer la tarde = at dusk / as evening falls

Both can relate to evening, but they are not exactly the same.

• la tarde = the afternoon / evening depending on context
• el atardecer = sunset / dusk / the time when evening falls

In caía la tarde, Spanish uses la tarde in an idiomatic way to mean the day was moving into evening.

If you used el atardecer, the meaning would shift slightly toward sunset itself or the dusk period.

So:

• caía la tarde = a broader, more atmospheric expression
• llegaba el atardecer or caía la noche = slightly different focus

The original sentence sounds very natural and elegant.

Here, volverse means to become.

So:

• el mar se volvía más frío = the sea was becoming colder

This is a very common use of volverse for change of state.

Without se, volver usually means to return or to turn back:

• Volvía a casa. = He/She was returning home.

So in this sentence:

• se volvía = was becoming
• volvía = would mean something completely different

Spanish has several verbs for to become, including:

• volverse
• ponerse
• hacerse
• llegar a ser

Here, volverse works well because it describes a change happening over time.

Both are possible in many contexts, but they are not always identical in tone.

• ponerse often suggests a change of state, sometimes temporary
• volverse often suggests a becoming or turning into something, sometimes with a more gradual or notable transformation

In this sentence, se volvía más frío sounds very natural because the sea is gradually changing as the evening falls.

You might also hear:

• el mar se ponía más frío

That is also understandable and natural. But se volvía gives a slightly smoother, more descriptive feel here.

So this is not a strict grammar rule so much as a matter of style and nuance.

Frío changes form because it is a regular adjective with masculine and feminine endings:

• el mar → masculine singular → más frío
• el agua → feminine singular → más fría

But agradable is an adjective with the same form for masculine and feminine singular:

• el aire agradable
• la brisa agradable

So in the sentence:

• el mar más frío
• el aire más agradable

both adjectives agree correctly with their nouns.

It is omitted to avoid repetition, but it is still understood.

The full version would be:

A medida que caía la tarde, el mar se volvía más frío y el aire se volvía más agradable.

Spanish often leaves out repeated material when it is clear from context. This is very natural.

So:

• el mar se volvía más frío y el aire más agradable

means:

• the sea was becoming colder and the air was becoming more pleasant

The second se volvía is simply understood.

Yes, but the meaning would be slightly different.

• se volvía más frío / más agradable focuses on change over time
• estaba más frío / más agradable focuses more on the resulting state

Compare:

• El mar se volvía más frío. = The sea was becoming colder.
• El mar estaba más frío. = The sea was colder.

Since the sentence begins with A medida que..., the idea of gradual change is important, so se volvía is the better choice.

Both are grammatically possible, but caía la tarde sounds more natural and idiomatic in this kind of descriptive writing.

Spanish often places the verb before the subject in literary or atmospheric expressions, especially with verbs describing the arrival or unfolding of natural events:

• caía la tarde
• llegó la noche
• soplaba el viento

This word order gives the sentence a more fluid, narrative feel.

So la tarde caía is not wrong, but caía la tarde is the more natural phrasing here.

Más agradable means more pleasant, which is broader and more subjective.

It suggests that the air felt nicer, more comfortable, or more enjoyable.

Other choices would change the meaning slightly:

• más fresco = cooler
• mejor = better
• más suave = milder / gentler

So:

• el aire más agradable does not just mean the air changed temperature
• it means the air became nicer overall

This is a good example of Spanish using an adjective that reflects perception, not just physical description.

It is natural, but it has a slightly descriptive or literary tone.

Especially these parts give it that feeling:

• A medida que...
• caía la tarde
• se volvía más frío

In everyday conversation, someone might say something simpler, like:

• Al atardecer, el mar se enfriaba y el aire era más agradable.
• Según anochecía, el mar estaba más frío y el aire se notaba mejor.

But the original sentence is absolutely correct and natural. It just sounds a bit more polished and scenic, like something from a story or a thoughtful description.

Se volvía is the imperfect tense of volverse.

It is used because the sentence describes an ongoing process in the past:

• the sea was becoming colder
• the air was becoming more pleasant

The imperfect is often used for:

• background description
• repeated or habitual actions in the past
• actions in progress
• gradual changes

That makes it perfect for this sentence, because everything is unfolding slowly as evening comes.

So both caía and se volvía help create a continuous, atmospheric scene rather than a sharp, finished event.