Learn Hardcore Portuguese (Portugal): A humidade da casa está alta, por isso abrimos as janelas para secar o ar. - The humidity in the house is high, so we open the windows to dry out the air.

In European Portuguese, it’s very common to use the definite article (o, a, os, as) with abstract nouns like humidade when you’re talking about a specific instance of that thing.

A humidade da casa está alta
= The humidity of the house is high.

Here, you’re not talking about humidity in general, but about the specific humidity in this house right now, so Portuguese naturally uses a.

You could technically say Humidade da casa está alta, but it sounds incomplete or ungrammatical to most native speakers. The article is normally required here.

Why is it humidade with an h? I’ve seen umidade without h.

The spelling depends on the variety of Portuguese:

European Portuguese (Portugal): humidade
Brazilian Portuguese: umidade

They are pronounced the same (the h is silent) and mean the same thing. Since you’re learning Portuguese from Portugal, humidade is the correct spelling for you.

Why is it está alta and not é alta?

Portuguese distinguishes between temporary and more permanent characteristics:

estar = temporary state, condition that can change
ser = more permanent or defining characteristic

Humidity is something that changes during the day, so it’s treated as a temporary condition:

A humidade da casa está alta.
The humidity in the house is (currently) high.

If you said:

A humidade da casa é alta.

it would suggest that the house tends to be humid by nature, as a general characteristic (for example, it’s always damp because it’s badly insulated). That’s possible in some contexts, but for a current situation, está alta is the natural choice.

What does da in da casa mean exactly?

Da is a contraction:

de + a = da

So da casa literally means “of the house” or “from the house.”

In this sentence:

A humidade da casa
= The humidity of the house
(i.e., the humidity in the house)

English often uses “in” here, but Portuguese often uses de to express “of / belonging to / associated with”:

a temperatura da água – the temperature of the water
o cheiro da cozinha – the smell of the kitchen

Could I say na casa instead of da casa?

No, na casa and da casa don’t mean the same thing:

da casa = de + a casa → “of the house”
- A humidade da casa = the house’s humidity / the humidity in the house
na casa = em + a casa → “in the house”
- A humidade na casa está alta = The humidity in the house is high

Both A humidade da casa está alta and A humidade na casa está alta are grammatical, but:

da casa sounds a bit more like “the house’s humidity” (more possessive/associative),
na casa emphasizes location (in the house).

In everyday speech, both might be used; da casa is very natural here.

Why do we say por isso and not porque in this sentence?

Por isso and porque play different roles:

porque = because (introduces a reason)
por isso = so / therefore (introduces a result)

In your sentence:

A humidade da casa está alta, por isso abrimos as janelas...
= The humidity is high, *so we open/opened the windows...*

You could also reverse the order and use porque:

Abrimos as janelas porque a humidade da casa está alta.
= We open/opened the windows *because the humidity is high.*

So:

porque points backwards to the cause.
por isso points forwards to the consequence.

How do I know if abrimos means we open (present) or we opened (past)?

In Portuguese, abrimos is the same form for:

present tense, 1st person plural:
- (nós) abrimos = we open
preterite (simple past), 1st person plural:
- (nós) abrimos = we opened

You know which one it is only from context:

Habit/general fact:
Quando a humidade da casa está alta, abrimos as janelas.
When the humidity is high, we open the windows. (present, habitual)
Specific past situation:
Ontem a humidade da casa estava alta, por isso abrimos as janelas.
Yesterday the humidity was high, so we opened the windows. (past)

In your standalone sentence, both readings are technically possible; usually learners meet it as a general strategy or as a specific past action depending on the wider context, which isn’t shown here.

Why is there no nós before abrimos?

In Portuguese, subject pronouns (eu, tu, ele, nós, etc.) are often omitted, because the verb ending already shows who the subject is.

Abrimos as janelas...
→ The -mos ending clearly indicates “we”.

You can include the pronoun for emphasis or contrast:

Nós abrimos as janelas, não eles.
We opened the windows, not them.

But normally, just abrimos is enough and more natural in neutral context.

Why do we say as janelas and not just janelas?

Portuguese uses definite articles (o, a, os, as) much more than English, especially with concrete nouns:

abrimos as janelas
= we open *the windows*

You’re talking about the specific windows of the house, known to both speaker and listener, so as is natural.

If you said just abrimos janelas, it would sound incomplete or strange in most contexts, as if you were talking about windows in general (and even then, it’s unusual).

Is para secar o ar the usual way to say “to dry the air”? It sounds a bit odd to me.

Grammatically it’s correct:

para
- infinitive (secar) = expresses purpose
  - para secar = in order to dry
o ar = the air

So:

abrimos as janelas para secar o ar
= we open the windows *to dry the air.*

However, in natural European Portuguese, people very often say it in a more idiomatic way, depending on what they mean:

para arejar a casa – to air the house
para arejar o ar – to freshen/air out the air
para secar a casa – to dry out the house (if it’s damp)
para reduzir a humidade – to reduce the humidity

Para secar o ar is understandable and not wrong, but some of the alternatives above may sound more idiomatic, depending on context.

Is o ar necessary, or could you just say para secar?

You can say just para secar, but it’s more vague:

abrimos as janelas para secar
= we open the windows to dry (things) / to dry it out

The listener would understand from context that you mean the air/the house is too humid, but grammatically it doesn’t say what is being dried.

para secar o ar – explicitly says dry the air
para secar a casa – dry the house
para secar – dry (unspecified “thing” / situation)

Is humidade always feminine? Is that why it’s alta and not alto?

Yes, humidade is a feminine noun in Portuguese:

a humidade – the humidity

Adjectives must agree in gender and number with the noun:

a humidade alta – the high humidity (feminine singular)
as humidades altas – the high humidities (rare, but grammatically)

If the noun were masculine, you’d use alto:

o preço alto – the high price (masculine singular)

Here it’s alta because it agrees with a humidade.

Can you explain how to pronounce humidade and por isso in European Portuguese?

Approximate pronunciation (European Portuguese):

humidade → [u-mi-DA-d(ɨ)]
- h is silent.
- Stress on DA: u-mi-DA-de.
- Final -de is often very reduced, sounding like a soft “d(uh)”.
por isso → roughly [pur EE-su] or [puɾ EE-su]
- por often sounds like “pur”.
- isso = EE-su (with a clear ee sound, then a light su).

Together in normal speech, it can flow like: [u-mi-DA-d(ɨ) pur EE-su].