Cotonou weather in September

Forum posters describe Benin's coast as having two rainy seasons, April through July and September through mid-October, both generally recommended to avoid given unstable weather and a rougher sea. Posters instead point to late December through mid-February as the better overall window for a Cotonou visit. Given July falls within the first of those two rainy stretches, posters' framing implies hot, humid conditions with substantial rain and choppier coastal waters typical of this part of the West African coast. Posters generally rate July as one of Cotonou's weaker weather windows, with the December-February dry stretch a clearly better-regarded alternative.

Forum posters describe the Harmattan winds - dry Saharan air carrying a lot of dust - as a defining feature of this stretch of the West African coast from roughly late December through mid-February, meaning January sits squarely within that hazy, dust-laden but comparatively dry window. Posters specifically flag February through April as the hottest months of the year here, implying January itself, while still warm and humid on the coast, runs a touch more moderate than the peak heat still to come. Posters generally recommend light, breathable clothing regardless of the Harmattan's dry-air reputation, since the dust itself - rather than rain - is the main practical annoyance travelers report for a January visit to Cotonou.