Climactic describes the high point, the most intense part of a movie, play, song, or, well, anything. Climatic refers to the climate, like the climatic changes that turned Santa's workshop into a sauna for elves.

Climactic comes from climax, with the x changed to a ct. A climax is the top point of something, so something climactic describes that intense moment. Climactic isoften used in the negative, anticlimactic, like when there's a build-up to something that falls short. But let's be positive:

But I have seldom seen this climactic moment staged to such haunting effect. (New York Times)

Still, a viewer would have to be made of stone not to get worked up during the stirring climactic section. (Reuters)

The skies were shattered with a final climactic crash of thunder. (Virginia Brooks)

Take out the extra "c," and climatic refers to weather conditions. The word climatic gets thrown around a lot these days, with all the talk about global warming, like the climatic insanity that made it snow on Halloween. FYI: There's no such word as  "anticlimatic." Also, climatic changes come from the climate, not the soil:

Warmer and wetter have become cooler and drier, a wholesale change of climatic fortunes. (Bakersfield Now)

This will trigger unpredictable climatic disasters in many parts of the world. (Huffington Post)

Lee eased Louisiana's worst drought since 1902, according to the National Climatic Data Center. (Business Week)

To keep climatic and climactic straight, remember that a climactic moment could be the most intense act yet. Something climactic gives you a kick! But climatic is just air.