Foz do Iguaçu has a subtropical climate with warm temperatures year-round — it rarely gets cold, but the wet season brings heavy rain. The falls are genuinely spectacular in all seasons, but the flow varies enormously.
High water (November to March) is when the falls are at their most violent and impressive. The Garganta do Diabo becomes a roaring wall of mist visible for kilometres. The downside is that this coincides with Brazil's summer holiday season — December to February — when accommodation prices spike and the national park gets crowded.
Low water (April to September) offers lower tourist numbers, cooler temperatures (15–25°C), and the ability to see individual falls more clearly rather than one continuous curtain of mist. You can sometimes walk out on rocks near the base. June and July are peak season for Argentine and Uruguayan visitors, so expect the Argentine side to be busy.
May and August are generally the sweet spot: the falls still run well, crowds are manageable, and prices are reasonable. Avoid the week of Carnival (February/March) unless you want to witness the festive chaos, and be aware that Brazilian school holidays in July bring families from across the country.