Atacama Desert Weather, Month by Month
Quick answer
San Pedro de Atacama receives about 38 mm of rain a year, more than half of it in February. Daytime highs run from 18.9 °C in June to 28.0 °C in December, January and February, and the night-time low is about 20 °C below the day's high in every month of the year.
Where these numbers come from
The Atlas Agroclimático de Chile, Universidad de Chile (2017), which is a station record for San Pedro de Atacama: temperatures 1958–1989, precipitation 1960–2015, humidity 2011–2015. We use it in preference to the gridded climate models most sites quote, because those models average over cells roughly 30 km wide that include the Andean slopes east of town. They put annual rainfall at 77 and 84 mm. The station says 38 mm. On a page about the driest desert on Earth, that difference is the subject.
The table
January 28.0/8.8 °C, 4 mm, 28% humidity. February 28.0/9.4, 21 mm, 38%. March 27.0/7.9, 7 mm, 31%. April 25.0/5.1, 0 mm, 23%. May 21.8/1.9, 1 mm, 22%. June 18.9/−0.5, 2 mm, 20%. July 19.3/−0.8, 0 mm, 18%. August 21.5/0.9, 1 mm, 14%. September 23.4/2.7, 1 mm, 13%. October 25.7/4.4, 0 mm, 13%. November 27.3/5.8, 1 mm, 17%. December 28.0/7.3, 0 mm, 22%.
The twenty-degree swing
The gap between the afternoon high and the night-time low is between 19 and 21 °C in every single month. January swings 19.2 degrees, July 20.1, October 21.3. There is no season in which you can pack for one temperature, and this is the fact that most affects what you actually do here.
The altiplanic winter
February takes 21 mm of the year's 38 mm and its humidity rises to 38 per cent. This is the invierno altiplánico, when moisture crosses the Andes from the Amazon basin and falls as afternoon storms on the high ground. It rarely rains on the town; it rains on the roads to El Tatio and the altiplanic lagoons, and tours get cancelled.
How dry it really is
Relative humidity bottoms out at 13 per cent in September and October. That is the practical reality of the desert: you lose water breathing, without ever feeling thirsty in proportion. It is also why the night sky here is what it is, because water vapour is what blurs starlight.
What the numbers do not include
El Tatio. The geyser field is at 4,320 m, nearly two kilometres above the station these figures come from, and operators report −10 to −5 °C there at dawn in every season. Nothing in this table describes that morning.
Frequently asked questions
How much rain does the Atacama get?
About 38 mm a year at San Pedro de Atacama according to the Chilean station record, with 21 mm of it falling in February alone.
How cold does it get at night?
Below freezing in June and July — −0.5 and −0.8 °C on average — and about 20 °C below the day's high in every month.
What is the driest month?
April, July, October and December all record 0 mm. Humidity is lowest in September and October at 13 per cent.
Does it ever rain in San Pedro?
Rarely, and mostly in February during the altiplanic winter. What that rain mainly does is close the roads to the high-altitude excursions.