Santiago has a cool semi-arid climate , with Mediterranean (Csb) patterns: warm dry summers (October to March) with temperatures reaching up to 35 °C (95 °F) on the hottest days; winters (April to September) are cool and humid, with cool to cold mornings; typical daily maximum temperatures of 14 °C (57 °F), and low temperatures near 0 °C (32 °F). In climate station of Quinta Normal (near downtown) the precipitation average is 341.8 mm, and in climate station of Tobalaba (in higher grounds near the Andes mountains) the precipitation average is 367.8 mm.
In the airport area of Pudahuel, mean rainfall is 276.9 mm (10.90 in) per year, about 80% of which occurs during the winter months (May to September), varying between 50 and 80 mm (1.97 and 3.15 in) of rainfall during these months. That amount contrasts with a very sunny season during the summer months between December and March, when rainfall does not exceed 4 mm (0.16 in) on average, caused by an anticyclonic dominance continued for about seven or eight months. There is significant variation within the city, with rainfall at the lower-elevation Pudahuel site near the airport being about 20 percent lower than at the older Quinta Normal site near the city center.
Santiago’s rainfall is highly variable and heavily influenced by the El Niño Southern Oscillation cycle, with rainy years coinciding with El Niño events and dry years with La Niña events. The wettest year since records began in 1866 was 1900 with 819.7 millimeters (32.27 in) – part of a “pluvial” from 1898 to 1905 that saw an average of 559.3 millimeters (22.02 in) over eight years incorporating the second wettest year in 1899 with 773.3 millimeters (30.44 in) – and the driest 1924 with 66.1 millimeters (2.60 in). Typically there are lengthy dry spells even in the rainiest of winters, intercepted with similarly lengthy periods of heavy rainfall. For instance, in 1987, the fourth wettest year on record with 712.1 millimeters (28.04 in), there was only 1.7 millimeters (0.07 in) in the 36 days between 3 June and 8 July, followed by 537.2 millimeters (21.15 in) in the 38 days between 9 July and 15 August.
Precipitation is usually only rain, as snowfall only occurs in the Andes and Precordillera, being rare in eastern districts, and extremely rare in most of the city. In winter, the snow line is about 2,100 meters (6,890 ft), and it ranges from 1,500–2,900 meters (4,921–9,514 ft).
The city is affected only occasionally by snowfall. The period between 2000 and 2017 has been registered 9 snowfalls and only two have been measured in the central sector (2007 and 2017). The amount of snow registered in Santiago on July 15, 2017 ranged between 3.0 cm in Quinta Normal and 10.0 cm in La Reina (Tobalaba).
Temperatures vary throughout the year from an average of 20 °C (68 °F) in January to 8 °C (46 °F) in June and July. In the summer days are very warm to hot, often reaching over 30 °C (86 °F) and a record high close to 38 °C (100 °F), while nights are very pleasant and cool, at 11 °C (52 °F). During autumn and winter the temperature drops, and is slightly lower than 10 °C (50 °F). The temperature may even drop to 0 °C (32 °F), especially during the morning. The historic low of −6.8 °C (20 °F) was in July 1976.
Santiago’s location within a watershed is one of the most important factors determining the climate of the city. The coastal mountain range serves as a screen that stops the spread of maritime influence, contributing to the increase in annual and daily thermal oscillation (the difference between the maximum and minimum daily temperatures can reach 14 °C) and maintaining low relative humidity, close to an annual average of 70%. It also prevents the entry of air masses, with the exception of some coastal low clouds that penetrate to the basin through the river valleys.
Prevailing winds are from the southwest, with an average of 15 km/h (9 mph), especially during the summer; the winter is less windy.