Learn Hardcore Turkish: Hava açık.

In Turkish, the present-tense copula (the equivalent of “is”) is usually dropped in simple adjectival or nominal statements. You don’t need to say Hava açıktır (“The weather is clear”) in everyday speech; Hava açık suffices. The suffix -tır/-dir can be added for formality or emphasis (giving Hava açıktır), but it’s optional.

What does açık literally mean, and how does it come to mean “clear” when talking about weather?

Literally, açık means “open.” By extension, when you describe the sky or weather as açık, you’re saying it’s “open” in the sense of not covered by clouds—hence “clear.” Many languages use the idea of openness to express clear skies.

What case are hava and açık in, and why aren’t there any suffixes on them?

Both words are in the nominative case (unmarked) because hava is the subject and açık is the predicate adjective. In a basic descriptive sentence, no additional case endings or possessive markers are needed.

Could I use a different Turkish word for “weather,” like iklim, instead of hava?

No—iklim means “climate,” referring to long-term atmospheric patterns, not day-to-day weather. For everyday conditions you must use hava. You could also say Gökyüzü açık (“The sky is clear”), but that emphasizes the sky rather than the general weather.