Copiapó is in the Atacama Desert, which means it rarely rains and temperatures vary significantly between day and night. Summer (December to March) is hot — daytime temperatures reach 30–35°C in the valley — but the high-altitude parks are accessible and the coastal beaches at Bahía Inglesa are at their most inviting. Winter (June to August) brings cooler days, cold nights, and occasional snow in the high Andes, making the mountain parks harder to reach but still possible.
The best time for the Parque Nacional Nevado Tres Cruces and Ojos del Salado is November to March, when the access roads are clear of snow and the weather is most stable for high-altitude driving. If you are hoping to see the Desierto Florido — the spectacular desert flowering — check conditions in August and September; it only happens in exceptional rainfall years.