Learn Hardcore Dutch: De lucht wordt zwart tijdens het onweer. - The sky turns black during the thunderstorm.

In this sentence, because lucht is becoming zwart (black), the natural interpretation is “the sky”.
If you talk about breathing or pollution, lucht usually means “air”:
- schone lucht – clean air
- vervuilde lucht – polluted air

Dutch often uses lucht where English uses sky, but there is also hemel, which can mean “sky” or “heaven” depending on context. For your example, de lucht is the normal, everyday choice.

What is wordt exactly, and why is it used instead of is?

Wordt is the 3rd person singular present form of the verb worden, which often means “to become”.

De lucht is zwart. – The sky is black. (state, description)
De lucht wordt zwart. – The sky becomes / turns black. (change of state)

So wordt here expresses a change happening, not just a static description. That’s why it’s used instead of is.

Why is wordt spelled with dt at the end?

Because it is:

present tense
3rd person singular (he / she / it → hij / zij / het)
of a verb with a stem ending in d (word- from worden)

Conjugation of worden in the present:

ik word
jij / je wordt
hij / zij / het wordt
wij / jullie / zij worden

The stem is word. For hij/zij/het, you add -t → wordt (d + t = dt). The d is not silent in spelling; it’s just that d and t sound almost the same at the end of words.

Could I also say De lucht is zwart tijdens het onweer? What’s the difference?

Yes, that sentence is grammatically correct, but the nuance is different:

De lucht wordt zwart… – emphasizes the process or change: “the sky turns black”.
De lucht is zwart… – describes a state: “the sky is black (at that time)”.

Your original sentence focuses on how, during the storm, the sky changes from some other color to black.

Why is it zwart and not zwarte?

Dutch adjectives only take -e in certain positions. The key rule:

Attributive (before a noun): usually with -e
- de zwarte lucht – the black sky
Predicative (after zijn, worden, blijven, etc.): no -e
- De lucht wordt zwart. – The sky becomes black.
- De lucht is zwart. – The sky is black.

In your sentence, zwart comes after the verb wordt and describes the subject (de lucht). This is predicative use, so it stays zwart, without -e.

What kind of word is tijdens, and how does it work in the sentence?

Tijdens is a preposition that means “during”. It is followed by a noun (or noun phrase):

tijdens het onweer – during the thunderstorm
tijdens de vakantie – during the holiday
tijdens de nacht – during the night

In your sentence, tijdens het onweer is a time phrase telling us when the sky turns black. It can appear:

at the end: De lucht wordt zwart tijdens het onweer.
or at the start: Tijdens het onweer wordt de lucht zwart.

Both word orders are correct; moving it changes focus slightly but not the basic meaning.

Why is it tijdens het onweer and not in het onweer or bij onweer?

These prepositions have different typical uses:

tijdens = during in a time sense
- tijdens het onweer – “during the thunderstorm” (while it is happening)
in = in/inside (place or abstract “within”)
- in de storm can sound like physically being inside the storm or in the middle of it
- in het onweer is unusual; Dutch normally says tijdens het onweer
bij often means “in case of / when there is / around the time of”
- Bij onweer moet je binnen blijven. – In case of a thunderstorm / When there is a thunderstorm, you must stay inside.

For describing what happens while the storm is occurring, tijdens het onweer is the most natural choice.

Why is it het onweer and not de onweer?

Again, this is about grammatical gender. Onweer is a neuter noun, so it takes het:

het onweer – the thunderstorm
een onweer – a thunderstorm

You have to learn nouns with their article:

het onweer, het weer (the weather), de regen (the rain), de wind (the wind)

There is no rule that tells you in advance that onweer is neuter; it’s mainly memorization and exposure.

What does onweer literally mean, and how is it different from storm?

Onweer is a noun meaning thunderstorm or stormy weather with thunder and lightning.

Etymologically, it’s related to weer (weather) and a prefix on-, but you don’t need to interpret it literally in modern Dutch; it’s just one word for thunderstorm.

Differences:

onweer – specifically thunderstorm (with thunder, usually lightning)
storm – a storm in general, often strong wind, not necessarily thunder
donder – thunder (the sound)
bliksem – lightning (the flash)

So tijdens het onweer means “during the thunderstorm”, not just any storm.

Can I change the word order to Tijdens het onweer wordt de lucht zwart?

Yes, that’s perfectly correct and very natural.

Dutch main clauses obey the V2 rule: the finite verb (here wordt) is in second position, but the first position can be something other than the subject, such as a time phrase:

De lucht wordt zwart tijdens het onweer.
Tijdens het onweer wordt de lucht zwart.

Both are grammatical; the second version just places more emphasis on “during the thunderstorm”.

Is wordt here a passive construction?

No. Here wordt is used as a linking verb meaning “becomes”.

Worden can do two jobs:

Linking verb (change of state):
- De lucht wordt zwart. – The sky becomes black.
Auxiliary for the passive voice:
- Het huis wordt gebouwd. – The house is being built.
  (Someone builds the house → passive)

In your sentence, there is no agent and no direct object that becomes the subject, so this is not passive; it’s simply “the sky becomes black”.

Is there also a verb related to onweer?

Yes, there is the impersonal verb onweren, which means “there is a thunderstorm” / “to storm with thunder”.

Examples:

Het onweert. – There is a thunderstorm / It’s thundering and stormy.
Als het onweert, wordt de lucht zwart. – When there is a thunderstorm, the sky turns black.

So you can talk about het onweer (the thunderstorm, as a noun) or het onweert (it is thundering / there is a thunderstorm, as a verb).