Climactic means you have reached a very exciting or important part of an event or story, especially near the end — the peak after everything has been building. The climactic scene of a film is the big fight or the big reveal; the climactic moment of a race is the last stretch. It comes from climax, the high point, plus -ic, formed to look like adjectives such as syntactic. The word is a little formal. In speech people often just say “the big moment” or “the climax.” It has nothing to do with weather, even though climatic looks almost the same and tricks a lot of readers.