The similar spelling here makes it so easy to use the wrong word, accidentally type the wrong word, or miss the incorrect usage when you are reading over your own work.

Climactic means ‘forming a climax’:

• The viewers gasped during the climactic final fight scene
• Mounting tension gave way to a climactic resolution

Climatic means ‘relating to climate or weather’:

• My hair doesn’t react well to these climatic conditions
• The researchers are worried about climatic change

My advice is to try to remember there is no extra c in climatic because climate does not have an extra c. Or there’s a more complicated association with climactic and cinema (because that’s where you often watch films with big climactic scenes).