Ночью ветер дует сильнее, чем днём.

In Russian, some time expressions use the instrumental case to mean at a certain time / during a period, especially with parts of the day or seasons.

So:

• ночь = night
• ночью = at night

This is a fixed and very common adverb-like use.

Compare:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

Even though instrumental often has other meanings too, here it simply tells you when something happens.

Both spellings may appear in writing, but днём is the version with the correct pronunciation marked by ё.

• день = day
• instrumental form: днём

The letter ё is often written as е in everyday texts, but it is still pronounced yo. So:

• written fully: днём
• often seen in less careful spelling: днем
• pronunciation: roughly dnyom

For learners, it is very helpful to remember the ё, because it shows both the sound and the stress.

Сильнее is the comparative form of сильный (strong).

• сильный ветер = a strong wind
• ветер дует сильнее = the wind blows more strongly / stronger

In English, we might expect an adverb like more strongly, but Russian often uses the comparative form here very naturally.

A few similar examples:

• быстро → быстрее = faster
• тихо → тише = more quietly / quieter
• громко → громче = louder

So сильнее, чем днём means stronger / more strongly than during the day.

Чем is the standard word for than in comparisons.

So:

• сильнее, чем днём = stronger than during the day

More examples:

• Он выше, чем я. = He is taller than me.
• Сегодня теплее, чем вчера. = It’s warmer today than yesterday.

This is one of the most common uses of чем, and it is very important for making comparisons.

Russian has no articles like a/an/the.

So ветер can mean:

• wind
• a wind
• the wind

Which one sounds best in English depends on context.

In this sentence, English often says the wind blows stronger at night than during the day, but Russian simply says ветер and leaves that detail to context.

Дует is the 3rd person singular present tense form of дуть (to blow).

Conjugation:

• я дую = I blow
• ты дуешь = you blow
• он/она дует = he/she/it blows

Since the subject is ветер (wind), which is singular, Russian uses дует:

• ветер дует = the wind blows

It can most naturally express a general fact / usual pattern:

• At night, the wind blows stronger than during the day.

Russian present tense often covers both:

1. what is happening now
2. what usually happens

Here, because of the time expressions ночью and днём, it sounds like a general observation or habit of nature.

Because днём by itself already works as a natural adverb meaning:

• during the day
• in the daytime

You do not need an extra noun like time.

Also, в день usually means something different, such as:

• per day
• on a day
• sometimes during the day, but in a different structure

For the contrast with ночью, Russian strongly prefers:

• ночью ... чем днём

This pair is very idiomatic.

Yes. Russian word order is more flexible than English, although the original version is the most neutral and natural:

• Ночью ветер дует сильнее, чем днём.

You could also hear:

• Ветер ночью дует сильнее, чем днём.
• Сильнее ветер дует ночью, чем днём.

But changing the order changes the focus or emphasis a little.

The original sentence is good because it starts with the time frame ночью and then states the fact clearly.

In practice, it describes the strength of the blowing, so in English we might translate it as more strongly.

Russian often uses forms like this without forcing a strict adjective/adverb distinction the way English does.

So:

• ветер сильнее = the wind is stronger
• ветер дует сильнее = the wind blows stronger / more strongly

Both are normal ways to express the idea.

A rough pronunciation guide:

• Ночью ≈ NO-chyu
• ветер ≈ VYE-tyer
• дует ≈ DOO-yet
• сильнее ≈ seel-NYE-ye
• чем ≈ chem
• днём ≈ dnyom

A few pronunciation notes:

• чью in ночью can feel tricky; it is a soft cluster.
• е after many consonants often sounds like ye.
• ё in днём is stressed and pronounced yo.
• сильнее has the stress on -нее: сильнЕе.

Yes, you could, but it changes the nuance slightly.

• Ночью ветер дует сильнее, чем днём.
  = At night the wind blows more strongly than during the day.

• Ночью ветер сильнее, чем днём.
  = At night the wind is stronger than during the day.

The difference is small, and both are natural. The version with дует makes the action of blowing more explicit.