Learn Hardcore Turkish: Hava güneşli görünüyor, oysa yağmur yağıyor. - The weather looks sunny, yet it is raining.

oysa ≈ “yet/whereas”: introduces a contrast that defies the expectation created by the first clause.
ama = plain “but”; neutral and very common.
fakat/ancak = “however”; a bit more formal/written.
halbuki ≈ “yet/in fact”; close to oysa, often a bit more emotive/argumentative.

Where can I place oysa, and do I need a comma? What about oysa ki?

Typical: at the start of the second clause: “..., oysa ...”.
It can also start a new sentence: “Oysa ...”.
Use a comma before it if you keep both clauses in one sentence.
oysa ki adds emphasis/emotion (“and yet…”); it’s optional here.

Why say Hava güneşli görünüyor instead of just Hava güneşli?

Hava güneşli states a fact (“the weather is sunny”).
Hava güneşli görünüyor says it merely looks sunny (your judgment from appearances), leaving room for a contradiction—perfect for the oysa clause that follows.

How is görünüyor formed, and why is it -üyor (not -iyor)?

Base: görünmek (“to appear; to be visible”), from görmek
- middle/reflexive -(I)n-.
Stem görün-
- present progressive -iyor with vowel harmony → -üyor (last vowel ü is front + rounded) = görünüyor.
3rd person singular has no extra ending.

Can I say Hava güneşli gibi görünüyor?

Yes. ... gibi görünüyor adds an explicit “as if” and can sound a bit more tentative. Without gibi, görünüyor already means “seems/looks.” Colloquially you can even say Hava güneşli gibi (elliptical for ... görünüyor).

Is gözüküyor interchangeable with görünüyor?

Largely, yes.

görünmek is the standard/neutral choice.
gözükmek is very common in everyday speech; some find it more colloquial.
Both usually work for “to be visible” and “to seem”: Hava güneşli gözüküyor ≈ Hava güneşli görünüyor.

Does Yağmur yağıyor sound redundant, like “rain is raining”?

No—it's idiomatic. yağmak is the verb used for precipitation.

Standard: Yağmur yağıyor (raining), Kar yağıyor (snowing), Dolu yağıyor (hailing).
Alternatives: Yağmur var (state: “there is rain”), Hava yağmurlu (adjectival: “it’s rainy”), or simply Yağıyor when context makes “rain” clear.

How do you pronounce yağıyor, and what does ğ do?

ğ (soft g) isn’t a hard consonant; it lengthens/smooths the preceding vowel.
Careful pronunciation: ya-ɯ-yor (note the dotless ı).
In fast speech it often sounds like “yaa-yor” because ğ lengthens the a and the ı can be very short.
So yağıyor is not pronounced like “yah-gee-yor.”

Can I drop the nouns hava or yağmur?

You can drop yağmur and say Yağıyor (“It’s raining”) if context is clear.
Dropping hava with görünüyor is less natural; keep a subject or replace it: Dışarısı güneşli görünüyor or Öyle görünüyor (“It seems so”).

Can I flip the clauses?

Yes:

Hava güneşli görünüyor, oysa yağmur yağıyor. (appearance → contradiction)
Yağmur yağıyor, oysa hava güneşli görünüyor. (fact → surprising appearance) Both are correct; the order just changes what you present first.

Should this be a semicolon instead of a comma?

Both are fine in Turkish:

Comma before oysa is common: ..., oysa ...
Semicolon gives a sharper split: Hava güneşli görünüyor; oysa yağmur yağıyor.
Or two sentences: Hava güneşli görünüyor. Oysa yağmur yağıyor.

Why not use the aorist, like görünür or yağar?

-yor = happening now.
görünür = “is (generally) visible/appears (as a property).”
yağar = habitual/scheduled (“In spring it rains”), not “it’s raining right now.”

What is oysa historically—one word or two?

It comes from o + ise (“as for that/if that is so”) fused into oysa. oysa ki adds emphasis: “and yet/however.”

What’s the difference between hava güneşli and güneşli hava here?

hava güneşli is a full clause: “the weather is sunny.”
güneşli hava is a noun phrase: “sunny weather.” Before görünüyor you need a clause, so Hava güneşli (görünüyor) is the natural structure.

Could I use yağmakta instead of yağıyor?

Yes. yağmakta is a more formal/reporting progressive (often in announcements): Şu anda yağmur yağmakta. In everyday speech, yağıyor is preferred.