Yağmur durunca sis oluşuyor.

-unca (or -ince after front vowels) is a temporal conjunctional suffix meaning “when,” “once,” or “after.” You attach it directly to the verb stem:
• dur- (stem of durmak, “to stop”) → dur-unca = “when it stops.”
It creates a subordinate clause describing the time frame for the main clause.

Both can express time, but with a slight nuance:
• durunca = “when/once it stops” (neutral, any time it stops)
• durur durmaz = “as soon as it stops” (emphasizes immediacy)
In our sentence, durunca simply states that fog forms whenever rain stops, without stressing an instant reaction.

In Turkish, durmak can be used with weather verbs to mean “to cease” or “to come to an end.” So:
• Yağmur durdu. = “The rain stopped.”
It’s a perfectly natural collocation, just like in English.

Turkish often uses the present progressive (-iyor form) for recurring events or natural phenomena:
• Sis oluşuyor. = “Fog is forming” (habitual/regular occurrence).
You could use the simple present (sis oluşur) to express a general truth, but it’s less common for weather processes. The progressive here emphasizes that fog regularly forms after the rain stops.

Yes. Turkish allows impersonal or zero subjects when the context is clear. Here, the subject is understood as “fog” or just an impersonal “it.” If you want to be explicit, you can say:
• Yağmur durunca sis oluşuyor. (implicit)
• Yağmur durunca sis oluşuyor. or even Sis oluşuyor yağmur durunca. (explicit sis)

Not really. Gelmek means “to come,” but for fog we usually use verbs like oluşmak (“to form”) or çökme k (“to settle, descend”):
• Sis oluşuyor. = “Fog forms.”
• Sis çökmeye başlıyor. = “Fog begins to settle.”