According to Christina Pamoni (see link below):
The effect of the weather and its spiritual inferences are mostly evident in Part I of ‘Things Fall Apart’. One of the fundamental beliefs of the Igbo tribe is the Chi that signifies the fate of a person. According to the African tradition, a person with weak Chi has to work really hard to succeed in life. For instance, Okonkwo’s father, Unoka had weak Chi and was viewed as a failure. Okonkwo, believing that he has inherited Unoka’s weak Chi, works very hard not to be considered a failure too. On the other hand, the idea of weak Chi serves as an excuse for Okonkwo when things do not work out as he hopes. He attributes any undesirable result to the bad Chi and avoids taking responsibility for his actions. When the drought destroys his harvest, he blames it on his unfortunate Chi; the next year that the crops survive, he praises his hard work. His will and determination help him to survive; his inability to alter his traditional beliefs destroys him.
In ‘Things Fall Apart’ rain and drought symbolize Okonkwo’s spiritual and emotional vanity. Rain is vital to the community of Umuofia. Without rain, families cannot be fed and people cannot be brought up normally. However, particularly in relation to Okonkwo, drought echoes his inner drought, an allegorical sterility in his heart. Focused on becoming better than his father, a natural born overachiever, he has no love for his son and he cannot express any kind of feeling for his family.
Moreover, the Igbo are a polytheistic society and they believe in particular gods for various aspects of nature. Often, they recognize the source of a drought as the consequence of a blasphemous act that has taken place on their soil. This is an allegory for Okonkwo. By killing – even accidentally – the young boy, Ikemefuna, he becomes blasphemous and this leads to his own emotional drought. The Igbo believe that to restore the rain and the land a sacrifice must take place, a self-punishment for sin. Okonkwo sacrifices himself by committing suicide. He offers himself to the save his village and atone the sins he visited upon the society.
According to Tenebris (see second link below):
The simplest reading of the weather in “Things Fall Apart” is on the level of dramatic irony: but even here, the weather not only reflects intense emotions but is intimately entangled with what it means to be human. How could it be otherwise? in a living world where supernatural powers are well- or ill-disposed toward a person based on his chi, his “personal god-force” or “fate”?
Even as Okonkwo, fearing to become his father Unoka, finds himself contrary and quick to anger at any perceived threat to his honour and work ethic, so the weather is constantly at odds with his drive to advance in his home village of Umuofia. Shame about his father has cast such a shadow of fear on Okonkwo that he himself grows into an impersonal, punishing agency of destruction not so very unlike the Igbo weather god. Achebe describes the sudden expression of Okonkwo’s suppressed anger as “the storm burst.” At one point early in the book, during the heart of the rainy season, the earth and sky are so entangled in gray wetness that it is uncertain whether Amadiora’s thunder comes from above or below. After Okonkwo chooses to kill Ikemefuna himself and starts to grow apart from his son, it becomes increasingly clear that the answer is both.
The weather reflects not only the storm in Okonkwo’s spirit, but the spiritual health of the entire village. Tribal lore and customs are intimately entangled with the village’s prosperity. Cultivation of the yam is carefully guided by festivals and proverbs, the result of which is usually good weather, a good harvest, and continued prosperity. As the rituals begin to be abandoned, a spiritual drought grows in the soul of Umuofia, a drought which also reverberates in the natural world.
Yet Umuofia has never been utterly dependent upon the vagaries of the weather. Although pre-colonial cultures are commonly assumed to be subsistence level, Okonkwo’s interactions with his fellow villagers reveal a sophisticated economic and social network where a determination to succeed is rewarded, and riches can be stored up despite repeated crop-damaging weather events. For those with ambition, dowries and title purchases, both methods of societal advancement, continually return large sums of money to the community in the form of village feasts and other gifts. Those without ambition, such as Okonkwo’s father, remain poor, low-status debtors, lower than women and sharing their name:
“Even as a little boy he had resented his father’s failure and weakness, and even now he still remembered how he had suffered when a playmate told him that his father was agbala. That was how Okonkwo first came to know that agbala was not only another name for woman, it could also mean a man who had taken no title.”
Although the village has the means to survive a poor harvest or two, paying heed to the voice of the weather remains important to survival. It is for this reason that, in the Igbo culture, weather remains one of the most important voices of the supernatural world. Even the village rain-maker will not interfere with the great cycles of the weather, either to call dry during the rainy season, or to summon rain in the heart of the dry season. Those who run counter to the great cycles of nature pay the price.
Agriculture is Umuofia’s most valued skill, and Okonkwo has excelled at it despite everything the weather could throw at him. “Yam, the king of crops, was a very exacting king” – and yet from a few borrowed yams, he has managed to build up enough wealth to purchase two titles and support three wives, and still have two barns full of yams. After he has survived that first terrible year, planting yams with borrowed seed and managing to eke out a harvest despite everything, Okonkwo knows that “[s]ince I survived that year, I shall survive anything.”
But it would be too easy to read weather simply as adversarial. Even while Okonkwo was still young, “he was already one of the greatest men of his time.” Had he not felt the need in himself to strive against the weather and the shadow of his father, had he not “worked daily on his farms from cock-crow until the chickens went to roost,” would his success, his fame, and his ultimate sacrifice have been as great? Okonkwo’s fame also comes from physical dominance, even from warfare. Okonkwo’s village is not peaceful, nor is it peaceful in its dealings with its neighbour villages; but the violence is ritualised to the point of broad societal stability. It is in adversity that great accomplishments can be achieved and fame can be gained.
Yet because Okonkwo is determined to succeed where his father had failed, he perseveres despite the contrary weather. Because Okonkwo believes he has inherited weak chi from his father, he knows that things will tend to go badly for him, so he must work even harder to succeed – and yet succeed he does. Even while just eighteen and still in his father’s shadow, Okonkwo defeated a great wrestler in one of the fiercest fights since the founding of the village. In the twenty years since then, his fame has continued to spread “like a bush-fire in the harmattan,” the dry winter wind. In Igbo belief, chi is not inevitability.
In the end Okonkwo kills himself rather than face colonial justice, and thereby severs himself from his people beyond eternity. In a world where the once-great people of Ani have lost all conviction, Okonkwo categorically rejects the Peace of Ani. He who had dragged yams all his life from an unwilling weather god and an unwilling Earth goddess will never now be one of the ancestors who dwell in close communion with her.
It is the ultimate rejection of both the receding dark shadow of his past and the looming white shadow in his future – a past and future which have already been swept away from his own people by the widening gyre of the approaching storm. Whether Okonkwo chose to be killed by the colonists or the path of self-annihilation, the end result is the same: no one will ever tend his shrine. His own son Nwoye is already among those who are letting the ancestors slip away. With the loss of their past, the people of Umuofia have themselves become the walking ghosts Ogbuefi Ezeudu had warned about among the Obodoani, “hungry to do harm to the living.” The Mother of the Spirits has already mourned their passing: “It seemed as if the very soul of the tribe wept for a great evil that was coming – its own death.”