Luften er kjølig om morgenen.

Because -en is the definite ending. Norwegian usually adds definiteness to the end of the noun instead of using a separate word like the.

• luft = air
• luften = the air

Even though English often treats air as an uncountable noun, Norwegian can still use the definite form naturally here, especially when talking about the air in a particular situation, like the morning air.

Kjølig is the adjective meaning cool or chilly, and here it is used after er as a predicate adjective.

In Norwegian, adjectives after å være still match the noun in number, and sometimes gender. Here, luften is singular, so kjølig is the correct singular form.

A useful extra point: adjectives ending in -ig often do not add -t in the neuter singular.

Examples:

• Luften er kjølig
• Været er kjølig
• Nettene er kjølige for plural

It means in the morning or sometimes in the mornings, depending on context.

In this sentence, it usually sounds like a general statement:

• The air is cool in the morning
• or The air is cool in the mornings

So it is often not about one single morning, but about what is generally true at that time of day.

Because om morgenen is a fixed and very common time expression in Norwegian. With parts of the day, Norwegian often uses om + definite form.

Compare:

• om morgenen = in the morning
• om kvelden = in the evening
• om natten = at night
• om vinteren = in winter

So morgenen is not unusual here; it is just the normal form in this kind of phrase.

They are completely different:

• om morgenen = in the morning
• i morgen = tomorrow

This is a very common point of confusion for learners.

So:

• Luften er kjølig om morgenen = The air is cool in the morning
• Luften er kjølig i morgen would mean The air is cool tomorrow, which is a different idea

Om has several meanings in Norwegian. One of them is a time meaning: during / in with certain time expressions.

So in this sentence, om does not mean about. It means something more like in or during:

• om morgenen = in the morning

This is one of those preposition uses that is best learned as a set expression.

Sometimes yes, and you may hear it, but om morgenen is the safest and most standard choice for a general meaning like in the morning.

A simple learner-friendly rule is:

• use om morgenen for general statements about that part of the day

So for this sentence, Luften er kjølig om morgenen is the best choice.

Yes. You can also say:

Om morgenen er luften kjølig.

That is perfectly natural. But when a time phrase comes first in a Norwegian main clause, the verb still has to stay in second position. That is why you get:

• Om morgenen er luften kjølig

and not:

• Om morgenen luften er kjølig

This is the usual Norwegian verb-second word order rule.

Yes, but the meaning changes slightly.

• kjølig = cool, chilly, mildly cold
• kald = cold

So:

• Luften er kjølig om morgenen suggests a fresh, cool feeling
• Luften er kald om morgenen sounds stronger and colder

If you want a milder, more natural description of morning air, kjølig is a very good choice.