tenkou ga kawatta.

It is read:

• 天候 → てんこう
• が → ga
• 変わった → かわった

So the full sentence is:

てんこう が かわった。

天候 means weather conditions or the weather. It often sounds a little more formal or objective than 天気.

• 天気 is the everyday word for weather
• 天候 can sound more like weather conditions, especially in reports, writing, or formal speech

So in casual conversation, people often say 天気が変わった.
Using 天候 is not wrong at all—it just sounds a bit more formal.

が marks 天候 as the thing that changed.

So:

• 天候 = the weather / weather conditions
• が = marks it as the subject
• 変わった = changed

A natural way to understand it is:

As for what changed? The weather did.

In simple sentences describing what happened, が is very common.

Yes, 天候は変わった is possible, but the nuance changes.

• 天候が変わった focuses on the fact that the weather changed
• 天候は変わった gives 天候 more of a topic feeling, like the weather, at least, changed or as for the weather, it changed

In many cases, が feels more natural when simply reporting an event.

変わった is the plain past form of the verb 変わる, which means to change.

• dictionary form: 変わる
• plain past: 変わった
• polite past: 変わりました

So 天候が変わった means the weather changed.

Because 変わる is a godan verb ending in -る, and its past form follows a sound-change pattern.

For verbs ending in -る / -う / -つ, the plain past usually becomes -った.

Examples:

• 変わる → 変わった
• 会う → 会った
• 待つ → 待った

So 変わった is the normal past form.

This is a very common question.

• 変わる = to change (something changes by itself, or is described as changing)
• 変える = to change something (someone changes it)

So:

• 天候が変わった = The weather changed
• 予定を変えた = I changed the plan

In grammar terms:

• 変わる is intransitive
• 変える is transitive

Japanese often leaves out anything that is already clear from context, but in this sentence the subject is actually stated: 天候.

What is missing is an English-style filler subject like it. Japanese usually does not need that.

English:

• The weather changed

Japanese:

• 天候が変わった

That is already a complete sentence.

天候が変わった。 is in the plain style.

That means it is common in:

• casual conversation
• narration
• diary-style writing
• informal statements

The polite version would be:

天候が変わりました。

Both mean the same thing, but 変わりました is more polite.

It can mean a broad kind of change, not just something physical.

With 天候, it means the weather conditions became different—for example:

• it was sunny, then it became cloudy
• the wind got stronger
• the conditions shifted in some noticeable way

So 変わった here means changed in a general sense, not necessarily a dramatic transformation.

The basic order here is very natural:

• 天候
  • が
    • 変わった

Japanese often places the verb at the end, so this is the standard pattern.

You may sometimes see extra words inserted before the verb, for example:

• 天候が急に変わった。 = The weather changed suddenly.
• 天候が大きく変わった。 = The weather changed significantly.

But the verb normally stays at the end.

Yes, but it depends on context.

天候が変わった is natural, especially in:

• weather-related discussion
• writing
• news or reports
• slightly formal speech

In everyday casual conversation, many speakers might more often say:

天気が変わった。

Both are correct.
The main difference is that 天候 sounds a bit more formal than 天気.