I would have to answer that the weather is not as significant as other factors in Okonkwo's downfall. The coming of the British and the changes they have wrought upon Ibo culture, in addition to Okonkwo's own stubbornness and arrogance, have a far greater impact on his life than natural phenomena. In fact, the only time weather plays a role is during times of drought. These dry seasons only strengthen Okonkwo's resolve. Although they create hardships for him and his family, he remains more determined than ever to prove a success. Yet this does not lead to his eventual suicide. That is a result of events over many years, mostly beyond Okonkwo's control. He could not stop the missionaries from coming to Umuofia, and try as he might, he could not stall their influence on members of the village. Some might argue that he himself could have changed, but the resoluteness was ingrained in his personality. So, it led directly to his tragedy.
Although it is not directly related to Okonkwo's situation, weather, particularly rain, is essential to the livelihood of the Ibo people. Without rain, the yams cannot grow, & without yams, a man cannot feed his family. Therefore he is not a man. For Okonkwo, the times of drought reflect a sterility in his heart. He feels no love for Nwoye, his first son, & he has difficulty expressing any kind of feelings for any other members of his family. These are the true causes of his downfall.