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Climate

San Francisco has a warm-summer Mediterranean climate (Köppen: Csb), characteristic of California's coast, with moist winters and dry summers. San Francisco's weather is strongly influenced by the cool currents of the Pacific Ocean on the west side of the city, and the water of San Francisco Bay to the north and east. This moderates temperature swings and produces a remarkably mild year-round climate with little seasonal temperature variation.

Among major U.S. cities, San Francisco has the coolest daily mean, maximum, and minimum temperatures for June, July, and August. During the summer, rising hot air in California's interior valleys creates a low-pressure area that draws winds from the North Pacific High through the Golden Gate, which creates the city's characteristic cool winds and fog. The fog is less pronounced in eastern neighborhoods and during the late summer and early fall. As a result, the year's warmest month, on average, is September, and on average, October is warmer than July, especially in daytime.

Temperatures reach or exceed 80 °F (27 °C) on an average of only 21 and 23 days a year at downtown and San Francisco International Airport (SFO), respectively. The dry period of May to October is mild to warm, with the normal monthly mean temperature peaking in September at 62.7 °F (17.1 °C). The rainy period of November to April is slightly cooler, with the normal monthly mean temperature reaching its lowest in January at 51.3 °F (10.7 °C). On average, there are 73 rainy days a year, and annual precipitation averages 23.65 inches (601 mm). Variation in precipitation from year to year is high. Above-average rain years are often associated with warm El Niño conditions in the Pacific while dry years often occur in cold water La Niña periods. In 2013 (a "La Niña" year), a record low 5.59 in (142 mm) of rainfall was recorded at downtown San Francisco, where records have been kept since 1849. Snowfall in the city is very rare, with only 10 measurable accumulations recorded since 1852, most recently in 1976 when up to 5 inches (13 cm) fell on Twin Peaks.

The highest recorded temperature at the official National Weather Service downtown observation station was 106 °F (41 °C) on September 1, 2017. During that hot spell, the warmest ever night of 71 °F (22 °C) was also recorded. The lowest recorded temperature was 27 °F (−3 °C) on December 11, 1932.

During an average year between 1991 and 2020, San Francisco recorded a warmest night at 64 °F (18 °C) and a coldest day at 49 °F (9 °C). The coldest daytime high since the station's opening in 1945 was recorded in December 1972 at 37 °F (3 °C).

As a coastal city, San Francisco will be heavily affected by climate change. As of 2021, sea levels are projected to rise by as much as 5 feet (1.5 m), resulting in periodic flooding, rising groundwater levels, and lowland floods from more severe storms.

San Francisco falls under the USDA 10b Plant hardiness zone, though some areas, particularly downtown, border zone 11a.