The epigraph of Nostromo gives us a clue that the book will present a pessimistic world-view and scenes of upheaval, but also a note of hope in the possible clearing of the "foul sky." The weather imagery (used as a metaphor even in the original Shakespeare) is fitting for a novel that starts, ends and is imbued with symbolic descriptions of its natural setting.
The epigraph is from Act IV scene 2 of King John, spoken by King John confronting a messenger who brings him news of invasion, after John has just heard with regret of the death of Arthur (murdered at his bequest), who is really still alive. The themes of invasion, bloodshed, repentance and error are all sounded in Nostromo as well.
Conrad puts the image of storms in political context for us early on, when
we are told that "the political atmosphere of the Republic was generally
stormy in these days."
Indeed, the novel will take us through three revolutions (the Ribierist, the
Monterist, and the War of Separation), with references to innumerable previous
wars from Caesar to Cortes to Garibaldi, and will leave us amid hints of a worker's
revolution yet to come. In this context, note what the quote really says: it
calls for foul weather as the solution to foul weather. Nostromo is a
novel about the "foul sky" of our political condition, and our endless
hope of clearing it by applying more of the same.