El Hierro's climate is mild year-round, but the island has distinct microclimates that vary sharply with altitude and aspect. The trade winds bring cloud and moisture to the northern and northeastern slopes — the laurel forests here stay lush even in summer — while the southern and western sides remain dry and sunny almost year-round.
Spring and autumn (March–May, October–November) are ideal: comfortable temperatures (20–25°C), calm seas, good diving visibility, and the island at its greenest after the winter rains.
Summer (June–September) brings more sun and warmer swimming, but also the calima — a hot wind from the Sahara that occasionally blankets the island in dust and pushes temperatures above 30°C. The calima usually lasts only a few days.
Winter (December–February) is the quietest season. Temperatures are mild at sea level (16–20°C) but cold at altitude. Rain falls mainly in short bursts. The diving is excellent in winter and visibility is often at its best.
The island is quiet by any standard — even in August, the number of visitors is small by Canary Islands measures. Accommodation is limited, so book ahead if you are visiting in summer.