Réunion has two distinct seasons. The austral winter (May to October) is dry, cooler, and the best time to visit. Temperatures in coastal towns range from 18°C to 26°C; in the mountains it can be considerably colder, especially at altitude at night. This is the ideal season for hiking — trails in the cirques are dry, views are clearer, and the risk of being caught in rain on an exposed ridge is much lower.
The austral summer (November to April) is hot, humid, and wet. Cyclones are possible from December through March. Heavy rainfall can close mountain roads and make some trails impassable or dangerous. The cirques — which trap cloud and moisture — are particularly wet. That said, the coast is still enjoyable, the sea is warmer, and accommodation is cheaper.
July and August are peak season, when mainland French and Mauritian visitors fill the island's gîtes and campgrounds. If you are planning to hike the GR R2 or stay at the Caverne Dufour refuge below Piton des Neiges, book well in advance.
The shoulder months of May–June and September–October offer a good compromise: reliable weather, reasonable prices, and fewer crowds on the trails.
The east coast, which faces the prevailing trade winds and receives most of the island's rainfall, is wetter year-round. The west coast, sheltered by the mountains, stays significantly drier regardless of season.