Terceira has a mild Atlantic climate year-round — temperatures rarely drop below 12°C in winter or rise above 26°C in summer. The island is rainier than you might expect: the Atlantic location and volcanic terrain combine to produce frequent showers even in summer, often passing quickly. June to September is the driest period and peak tourist season; this is also when the tourada à corda and the major summer festivals (including the Sanjoaninas in late June) take place.
Spring (April–May) is green, quieter, and often lovely. The hydrangeas that line the island's roads are in full bloom in June. Winter is mild but often grey and wet; the cave at Algar do Carvão and the museums of Angra are the sensible targets.