Nasłonecznienie jest wysokie.

Not exactly. Nasłonecznienie is a more technical or formal word meaning sun exposure, amount of sunlight, or insolation.

So Nasłonecznienie jest wysokie means something like:

• Sun exposure is high
• The amount of sunlight is high
• Insolation is high in a technical context

It is not usually the everyday word for sunshine in the simple weather sense. For that, Polish often uses words like słońce or phrases such as jest dużo słońca.

A helpful breakdown is:

na-sło-necz-nie-nie

A rough guide:

• na = like na in nacho
• sło = the ł sounds like English w, so this part sounds roughly like swo
• necz = the cz sounds like ch in chocolate
• nie = roughly nye

The stress is on the second-to-last syllable:

na-sło-necz-NIE-nie

A rough approximation could be:

nah-swo-NECH-NYEH-nyeh

But it is better to remember these key points:

• ł = w
• cz = ch
• nie often sounds like nye

It is neuter.

A big clue is the ending -nie. Many Polish abstract nouns ending in -nie are neuter.

Because nasłonecznienie is neuter singular, the adjective must also be neuter singular:

• nasłonecznienie → neuter singular
• wysokie → neuter singular form of wysoki

That is why the sentence is:

Nasłonecznienie jest wysokie.

and not wysoki or wysoka.

Because adjectives in Polish must agree with the noun in gender, number, and usually case.

Here:

• nasłonecznienie is neuter singular
• so the adjective must also be neuter singular
• the neuter singular form of wysoki is wysokie

Compare:

• wysoki budynek = a tall building masculine
• wysoka temperatura = a high temperature feminine
• wysokie nasłonecznienie = high sun exposure neuter

So wysokie is required by grammar.

It is in the nominative case because it is the subject of the sentence.

The structure is:

• Nasłonecznienie = subject
• jest = is
• wysokie = predicate adjective describing the subject

So the sentence literally works like:

Sun exposure + is + high

The adjective wysokie matches the subject.

In normal standard Polish, yes, you need it.

Polish usually keeps the present-tense form of być in ordinary sentences like this:

• Nasłonecznienie jest wysokie.

If you remove jest, you get:

• Nasłonecznienie wysokie

That sounds incomplete, very elliptical, or like a note, heading, chart label, or headline rather than a full normal sentence.

So for regular speech and writing, use jest.

The most neutral and natural order is:

Nasłonecznienie jest wysokie.

Polish word order is flexible, so other orders are possible, but they sound more marked or stylistic. For example:

• Wysokie jest nasłonecznienie.
  This sounds unusual and emphasizes wysokie.

• Jest wysokie nasłonecznienie.
  This is possible in some contexts, but it is less natural as a plain statement.

For a learner, the safest choice is the original sentence order.

Yes. In Polish, wysokie nasłonecznienie is a normal collocation, especially in more formal, scientific, geographic, architectural, agricultural, or environmental contexts.

Polish often uses high with measurable abstract nouns, just like English does with things such as:

• high temperature
• high humidity
• high pressure

So wysokie nasłonecznienie is perfectly natural.

It sounds a bit formal or technical.

You might see it in contexts like:

• weather reports
• climate descriptions
• gardening
• agriculture
• architecture
• solar energy
• geography

In everyday speech, people might choose something simpler depending on what they mean, for example:

• Jest dużo słońca. = There is a lot of sun.
• To miejsce jest bardzo słoneczne. = This place is very sunny.
• To mieszkanie jest dobrze nasłonecznione. = This apartment gets a lot of sunlight.

So the original sentence is correct, but not the most casual option.

They are related, but not the same.

• Nasłonecznienie jest wysokie talks about the level/amount of sunlight in a more abstract way.
• To miejsce jest nasłonecznione means this place is sunlit / gets good sunlight and describes a specific place more directly.

So:

• nasłonecznienie = a noun, the phenomenon or level of sun exposure
• nasłonecznione = an adjective, describing something as sunny or well-exposed to sunlight

If you are talking about a room, garden, balcony, or apartment, the adjective version is often more natural.

To make it negative, put nie before jest:

• Nasłonecznienie nie jest wysokie. = The sun exposure is not high.

To make it a yes/no question, you usually just use intonation in speech:

• Nasłonecznienie jest wysokie? = Is the sun exposure high?

In writing, Polish often keeps the same word order for simple yes/no questions.

You could also ask more explicitly:

• Czy nasłonecznienie jest wysokie? = Is the sun exposure high?

Using czy is very common and clear for learners.

Sometimes you may encounter both, but wysokie nasłonecznienie is the more standard and expected collocation.

Why? Because nasłonecznienie is treated as a measurable level or intensity, and Polish often uses wysoki for that kind of noun.

So as a learner, wysokie nasłonecznienie is the safest and most natural choice.