Learn Hardcore Greek: Αύριο δεν θα έχει χιόνι και η θερμοκρασία θα είναι καλύτερη για βόλτα. - Tomorrow there will be no snow and the temperature will be better for a walk.

The phrase θα έχει χιόνι is impersonal and literally means “will have snow”, but it is understood as “there will be snow / it will have snow”. Greek does not use a dummy subject like English “it” or “there” in such sentences.

So:

(Αύριο) δεν θα έχει χιόνι ≈ “(Tomorrow) there won’t be snow / it won’t have snow.”

No extra word for “it” is required or used.

Why is the negative δεν placed before θα and not after it?

In modern Greek, the normal placement is:

δεν + θα + verb

The negative particle δεν goes directly before the future particle θα, which together count as part of the verb phrase:

δεν θα έχει – “will not have”
δεν θα είναι – “will not be”

You don’t say *θα δεν έχει or *θα έχει δεν.
So the correct order in your sentence is:

Αύριο δεν θα έχει χιόνι…

What exactly does θα έχει mean here, and how is the future tense formed?

Θα έχει comes from the verb έχω (“to have”) with the future particle θα:

έχω – “I have”
θα έχω – “I will have”
θα έχει – “he/she/it will have” or impersonal “there will be”

In modern Greek, the usual way to form the future is:

θα + present tense of the verb

So:

θα έχει – “will have / there will be”
θα είναι – “will be”

In weather or existence expressions, θα έχει is often best translated as “there will be”, not literally “it will have”.

What is the difference between χιόνι and χιονίζει?

χιόνι is a noun: “snow”.
χιονίζει is a verb: “it is snowing” / “it snows”.

In your sentence:

δεν θα έχει χιόνι – “there will not be snow”

You could also use the verb:

Αύριο δεν θα χιονίσει. – “Tomorrow it will not snow.”
Αύριο δεν θα χιονίζει. – “Tomorrow it won’t be snowing.”

δεν θα έχει χιόνι focuses more on the presence of snow (on the ground / generally),
δεν θα χιονίσει focuses on the event of snowfall.

In everyday speech, all of them are common, with slightly different nuances.

Why do we say η θερμοκρασία with the article η? In English we just say “temperature”.

Greek uses the definite article much more often than English.

Here, η θερμοκρασία literally is “the temperature”, but it usually means “the (outside) temperature” or “the air temperature” – something specific and known from context.

Some patterns:

η θερμοκρασία είναι χαμηλή – “the temperature is low”
η μουσική είναι δυνατά – literally “the music is loud”, often just “the music’s loud”

So: > …και η θερμοκρασία θα είναι καλύτερη…
> “…and (the) temperature will be better…”

is the natural Greek way to say what English expresses simply as “the temperature / the weather will be better”.

Why is it καλύτερη and not καλύτερο or καλύτερος?

Καλή / καλύτερη is an adjective and must agree in gender, number, and case with the noun it describes.

η θερμοκρασία – feminine singular
So the adjective must also be feminine singular: καλύτερη

Main forms:

masculine: καλύτερος
feminine: καλύτερη
neuter: καλύτερο

Examples:

ο καιρός είναι καλύτερος – the weather (masc.) is better
η θερμοκρασία είναι καλύτερη – the temperature (fem.) is better
το κλίμα είναι καλύτερο – the climate (neut.) is better

Could I say η θερμοκρασία θα είναι πιο καλή instead of καλύτερη?

You can say πιο καλή, and it’s grammatically correct, but it sounds less natural in this context.

Καλύτερος / καλύτερη / καλύτερο is the normal comparative form of καλός / καλή / καλό (“good”). It corresponds to English “better”.

καλός → καλύτερος
καλή → καλύτερη
καλό → καλύτερο

πιο καλή literally means “more good” and is usually avoided when there is a common, simple comparative form like καλύτερη. Native speakers overwhelmingly prefer:

η θερμοκρασία θα είναι καλύτερη

What does the phrase για βόλτα mean exactly, and why is there no article?

Βόλτα is a feminine noun meaning “walk, stroll, outing”.

With για, the phrase για βόλτα means “for (going on) a walk / for a stroll” in a general, activity sense.

The article is often omitted after για when we talk about activities in general:

για βόλτα – for walking / for a walk (in general)
για ύπνο – for sleep(ing)
για καφέ – for coffee (to have a coffee)

If you say:

για μια βόλτα – “for a (specific) walk”
για τη βόλτα – “for the walk” (a particular walk)

So για βόλτα here means “suitable for walking in general”, not for one particular planned walk.

Why is the preposition για used here? Could we use σε instead?

Here, για means “for / suitable for / intended for”:

…η θερμοκρασία θα είναι καλύτερη για βόλτα.
“…the temperature will be better for a walk.”

Για + noun often expresses purpose, suitability, or destination:

καλός για φαγητό – good for food
ιδανικό για παιδιά – ideal for children

Σε mostly means “in / at / to”, indicating location or direction:

πάω σε βόλτα – not used
πάω βόλτα – I go for a walk (no preposition)
πάω σε ένα πάρκο – I go to a park

So in this sentence only για βόλτα is natural.

Can we change the word order, for example put αύριο later or move η θερμοκρασία?

Yes. Greek word order is more flexible than English. The original:

Αύριο δεν θα έχει χιόνι και η θερμοκρασία θα είναι καλύτερη για βόλτα.

You could also hear:

Δεν θα έχει χιόνι αύριο και η θερμοκρασία θα είναι καλύτερη για βόλτα.
Αύριο δεν θα έχει χιόνι και θα είναι καλύτερη η θερμοκρασία για βόλτα.

All are grammatical. The differences are mostly in emphasis and rhythm, not in basic meaning. The original version is very natural and neutral in style.