L’humidité ambiante est basse.

Because humidité begins with an h muet in French. That means the h is not pronounced and behaves like a vowel at the beginning of the word, so la becomes l’ before it.

So:

• la humidité → not correct
• l’humidité → correct

This is the same pattern as:

• l’homme
• l’histoire
• l’habitude

It is feminine: l’humidité.

You can tell in this sentence because the adjective later is basse, which is the feminine form of bas. So the sentence shows agreement with a feminine noun.

Many French nouns ending in -ité are feminine, for example:

• la possibilité
• la qualité
• la vitesse

Ambiante means ambient, surrounding, or in the surrounding environment/air.

So l’humidité ambiante means the humidity in the surrounding air or environment, not moisture inside a specific object or material.

It is a fairly standard, neutral expression, especially in technical, scientific, weather-related, or indoor-environment contexts.

In French, many adjectives come after the noun, and ambiant / ambiante normally does.

So:

• l’humidité ambiante = the ambient humidity

This is very normal French word order. English often puts adjectives before the noun, but French often places them after.

Because adjectives agree with the noun they describe.

Here, humidité is feminine singular, so the adjective must also be feminine singular:

• masculine singular: bas
• feminine singular: basse

Even though basse comes after est, it still agrees with l’humidité. French predicate adjectives also agree:

• Le niveau est bas.
• L’humidité est basse.

Because French uses être + adjective to describe a state or condition.

Here, basse is an adjective describing the humidity level, so French says:

• L’humidité ambiante est basse.

Not:

• a basse

This works like:

• La température est élevée.
• La pression est faible.
• La pièce est froide.

Yes, often you can say L’humidité ambiante est faible.

Both basse and faible can mean low here, but there is a slight nuance:

• basse often emphasizes a low level on a scale
• faible often means weak, small in amount, or low

In many practical contexts, both are natural.
Humidité basse and humidité faible are both understandable, but humidité basse sounds very natural when talking about a measured level.

A simple pronunciation guide is:

L’humidité ambiante est basse
≈ ly-mee-dee-tay ahn-byahnt ay boss

A few helpful points:

• l’ links directly to humidité
• the h in humidité is silent
• ambiante has a nasal sound in -am-
• est is pronounced ay
• basse sounds like boss, not like the English word base

Usually, yes: between est and basse.

So speakers often say:

• est basse → ay-bass

But there is normally no liaison between ambiante and est, because ambiante ends with a written e, not with a normally pronounced liaison consonant.

It is neutral and a little technical/scientific in tone.

You might see or hear it in:

• weather reports
• building/environment discussions
• appliance manuals
• lab or industrial contexts

In everyday speech, someone might also say something like:

• Il y a peu d’humidité dans l’air.
• L’air est sec.

Those can sound more conversational depending on context.

Not really in this sentence. Here basse clearly means low in level or amount.

The adjective bas / basse can also mean:

• physically low: une table basse
• lower in position: une étagère basse
• low-pitched: une voix basse can mean a quiet/low voice depending on context

But with humidité, the intended meaning is the level is low.

Often yes, depending on what you want to emphasize.

• L’humidité ambiante est basse is more precise and technical.
• L’air est sec is more natural in everyday conversation and focuses on how the air feels.

They are often close in meaning, but not always perfectly identical:

• low ambient humidity is a measured condition
• the air is dry is a more general description

They do different jobs:

• ambiante directly modifies the noun humidité
  → l’humidité ambiante = the ambient humidity

• basse comes after être and describes the state of that noun phrase
  → est basse = is low

So the structure is:

[noun + adjective] + être + adjective
L’humidité ambiante + est + basse

In other words:

• ambiante tells you which humidity
• basse tells you what it is like