The Atacama is one of the driest places on Earth, so weather is rarely a deciding factor. Year-round, days are dry, clear, and dusty; nights at this altitude (2,250 m) can be sharply cold. Summer (December–March) is hottest, with daytime temperatures around 25–28°C and warm evenings; this is also the brief season of the so-called "Bolivian winter", when occasional thunderstorms cross the highlands and bring short rains.
Winter (June–August) is dry and cool — pleasant by day, near-freezing at night. The shoulder months of April–May and September–October are arguably the best time, with stable weather and lower visitor numbers. Avoid hitting Calama around the Independence Day holiday (18 September), when flights and onward transport to San Pedro fill up weeks in advance.