The Chaco has two seasons. From October to April it is hot — frequently above 40°C, sometimes above 45°C — and humid, with violent thunderstorms that can leave the dirt roads impassable for days. From May to September it is cooler (daytime highs in the high 20s, nights occasionally near freezing), the air is dry, and the roads stay open.
The dry season is by far the better time to visit. Animals concentrate at the surviving water holes, the heat is bearable, and the famous "Trans-Chaco Rally" — a punishing off-road race held in late September — turns Filadelfia and its sister towns into the noisy centre of Paraguayan motorsport for a weekend.