La Serena sits in a transitional climate zone — drier and warmer than Santiago, cooler and wetter than the Atacama. The weather is mild year-round, hovering between 12°C in winter and 24°C in summer, with most rain falling in June and July.
The Chilean summer (January and February) is high season: the beach fills, prices rise, and the observatories book out weeks ahead. November to March is the better window for clear skies in the Elqui Valley. May to August can be grey on the coast and is the worst time to come for sky-watching, but the towns themselves are pleasant and uncrowded.