San Francisco has a warm-summer Mediterranean climate characteristic of California’s coast, with moist mild 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.
Because of its sharp topography and maritime influences, San Francisco exhibits a multitude of distinct microclimates. The high hills in the geographic center of the city are responsible for a 20% variance in annual rainfall between different parts of the city. They also protect neighborhoods directly to their east from the foggy and sometimes very cold and windy conditions experienced in the Sunset District; for those who live on the eastern side of the city, San Francisco is sunnier, with an average of 260 clear days, and only 105 cloudy days per year.
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. The lowest recorded temperature was 27 °F (−3 °C) on December 11, 1932. The National Weather Service provides a helpful visual aid graphing the information in the table below to display visually by month the annual typical temperatures, the past year’s temperatures, and record temperatures.
San Francisco falls under the USDA 10b Plant hardiness zone.