Při rosení je vzdušná vlhkost vysoká.

Here při means something like during, while, or in the course of.

So při rosení sets the situation or time frame: during rosení / when rosení happens.

A key grammar point: při normally requires the locative case.

It is in the locative singular, because the preposition při takes the locative.

The tricky part is that rosení looks the same as its basic dictionary form. That is normal: many neuter verbal nouns ending in -ní / -tí have the same form in the nominative, accusative, and locative singular.

So:

• dictionary form: rosení
• after při: při rosení

Same spelling, different grammatical role.

It is a noun, specifically a verbal noun.

It comes from the verb family rosit / rosit se, but in this sentence it behaves like an ordinary noun. That is why it can follow a preposition:

• při rosení

So even though it is connected to an action or process, grammatically it is not a finite verb here.

The subject is vzdušná vlhkost.

The structure is:

• vzdušná vlhkost = subject
• je = linking verb
• vysoká = predicate adjective

So the sentence is built like:

• X is Y
• Vzdušná vlhkost je vysoká

Because both words agree with vlhkost, which is feminine singular.

• vzdušná describes vlhkost
• vysoká also agrees with vlhkost after je

So both adjectives take the feminine singular nominative ending -á.

Because in Czech, many feminine nouns end in a consonant, especially abstract nouns ending in -ost.

This is a very common pattern. For example:

• radost = joy
• rychlost = speed
• možnost = possibility
• vlhkost = humidity

So vlhkost is feminine even though it does not have the typical -a ending.

In normal standard Czech, je should be there.

Czech usually keeps the present tense of být in sentences like this:

• Vzdušná vlhkost je vysoká.

Leaving it out would sound incomplete in ordinary prose. You might see omission only in very special styles, such as notes, headlines, or poetry.

Yes, Czech word order is flexible.

For example, these are both grammatical:

• Při rosení je vzdušná vlhkost vysoká.
• Vzdušná vlhkost je při rosení vysoká.

The version with Při rosení first gives more emphasis to the circumstance or time setting. That is very natural in explanatory or technical writing.

Both are possible, and they are very close in meaning.

• vzdušná vlhkost = a fixed expression, somewhat technical
• vlhkost vzduchu = literally humidity of the air

So this is mostly a matter of style and collocation. Vzdušná vlhkost is a normal established phrase.

Not always. Its exact meaning depends on context.

Depending on the situation, rosení can refer to things like:

• dew formation / condensation
• misting / spraying lightly

So the surrounding context tells you which meaning is intended. Grammatically, though, the word works the same way.

Yes. Czech uses vysoký / vysoká / vysoké not only for physical height, but also for levels, amounts, or intensity.

So vysoká vlhkost is perfectly normal and means high humidity.

Compare:

• vysoká teplota = high temperature
• vysoký tlak = high pressure
• vysoká vlhkost = high humidity

Not necessarily.

By itself, vzdušná vlhkost means humidity of the air in a general sense. If you want to say relative humidity specifically, Czech usually says:

• relativní vlhkost vzduchu

So this sentence does not automatically mean relative humidity unless the broader context makes that clear.

Because Czech has no articles.

English needs words like a, an, and the, but Czech normally does not use separate words for that. Definiteness is understood from context.

So Czech simply says:

• Při rosení je vzdušná vlhkost vysoká.

without any direct equivalent of the or a.