Što je nadmorska visina veća, to je vreme hladnije.

It is a fixed Serbian pattern used to show a proportional relationship:

• Što je X veći/veća/veće, to je Y veći/veća/veće
• English: the more / the bigger / the higher X is, the more / the bigger / the higher Y is

So here, što and to do not mean their usual dictionary meanings like what or that. Together, they form the equivalent of English the ..., the ... in sentences like:

• The higher the altitude, the colder the weather.

Veća is the comparative form of velika / veliko / velik in the sense of greater / bigger.

It agrees with nadmorska visina, which is:

• feminine
• singular
• nominative

So:

• visina = height, altitude
• veća visina = greater altitude

In the sentence, veća is a predicate adjective after je, so literally:

• nadmorska visina je veća = the altitude is greater

That is a very natural question, because English often says higher altitude.

In Serbian, with nadmorska visina, veća is very common and standard, because the altitude is being treated as a measured value: greater altitude.

You may also hear viša in some contexts, but veća nadmorska visina is the safer and more standard choice for learners.

So:

• veća nadmorska visina = greater altitude
• English may prefer higher altitude, but Serbian often uses greater

Because it agrees with vreme, and vreme is a neuter singular noun.

So:

• vreme = neuter singular
• hladnije = comparative adjective in neuter singular

That is why you get:

• vreme je hladnije = the weather is colder

Compare:

• dan je hladniji = the day is colder
• noć je hladnija = the night is colder
• vreme je hladnije = the weather is colder

Both are in the nominative singular.

That is because each clause has its own subject:

• nadmorska visina je veća
• vreme je hladnije

So:

• nadmorska visina = nominative singular, subject of the first clause
• vreme = nominative singular, subject of the second clause

Because this sentence is made of two full clauses, and each clause has its own verb:

1. Što je nadmorska visina veća
2. to je vreme hladnije

In other words:

• nadmorska visina je veća
• vreme je hladnije

The što ..., to ... structure links the two clauses, but it does not remove the need for the verb biti (to be) in each one.

Not completely. Serbian word order is fairly flexible, and you may also hear:

• Što je veća nadmorska visina, to je vreme hladnije.

This is also natural and maybe even slightly smoother to many speakers.

The original sentence is fully correct, but Serbian often allows the comparative adjective to come before the noun more easily than English does.

So both of these are fine:

• Što je nadmorska visina veća, to je vreme hladnije.
• Što je veća nadmorska visina, to je vreme hladnije.

Because standard Serbian punctuation separates the two parts of this correlative structure:

• Što ..., to ...

So the comma marks the boundary between the two linked clauses.

This is the normal written form:

• Što je nadmorska visina veća, to je vreme hladnije.

Vreme is the Ekavian Serbian form.
Vrijeme is the Ijekavian form.

Both belong to Serbian, depending on regional and standard variety.

So in Serbian you may see:

• vreme
• vrijeme

The sentence in Ijekavian Serbian would be:

• Što je nadmorska visina veća, to je vrijeme hladnije.

The grammar is the same; only the pronunciation/spelling variety changes.

Yes, it is natural and correct.

That said, if the speaker wants to be more precise or scientific, they might say:

• Što je nadmorska visina veća, temperatura je niža.
• The greater the altitude, the lower the temperature.

Why? Because strictly speaking, altitude directly affects temperature, while vreme means weather more generally.

So:

• ... vreme hladnije = natural, general statement
• ... temperatura niža = more precise, scientific style

Not really. Serbian does not use an article-based pattern like English the ..., the ....

Instead, it uses the fixed correlative structure:

• što ..., to ...

So the natural Serbian way is not a word-for-word translation, but a structural equivalent:

• Što je nadmorska visina veća, to je vreme hladnije.

This is a good example of a sentence where you should learn the whole pattern, not translate each English word separately.