San Francisco occupies a unique position on the northern California coastline, profoundly influencing its weather patterns. The city’s climate is classified as Mediterranean, defined by mild, wet winters and warm, significantly dry summers. Understanding the total amount of precipitation the city receives annually requires looking closely at how this moisture is delivered throughout the year. This analysis provides data on the typical rainfall totals for this coastal city.
The Average Annual Precipitation Total
The official long-term average annual precipitation recorded for San Francisco is approximately 23.65 inches (601 millimeters). This measurement, often taken at the downtown weather station, serves as the baseline for comparing annual moisture totals.
San Francisco’s annual rainfall is moderate compared to other major coastal cities in the United States. For instance, Seattle, Washington, typically records about 38 inches (965 mm) annually. Conversely, a drier Southern California counterpart, downtown Los Angeles, averages closer to 14.93 inches (379 mm) per year. The 23.65-inch average places San Francisco squarely in the middle of this range, reflecting its distinct climate zone.
The average total is calculated by compiling decades of data to smooth out the large variations that occur between years, especially in a region prone to cycles of drought and heavy rain. This standard measurement provides a reliable benchmark for understanding the city’s overall water budget.
Seasonal Distribution of Rainfall
The total annual rainfall is heavily skewed toward a specific six-month window, a defining characteristic of the Mediterranean climate. Nearly 90% of the city’s precipitation occurs between November and April.
This concentration is due to the North Pacific High-Pressure System, which blocks storm tracks from reaching the California coast during the summer. Once this high-pressure system weakens and moves south in the autumn, Pacific storms begin to bring significant moisture inland.
The three wettest months are typically December, January, and February, which collectively account for a substantial portion of the yearly total. During this peak winter period, the average monthly rainfall often exceeds three to four inches.
In stark contrast, the summer months of June, July, and August are statistically very dry. The combined precipitation for this entire three-month period rarely totals more than 0.25 inches, with many years recording zero measurable rainfall. This extreme seasonal variation requires the city’s infrastructure and ecosystem to manage a large influx of water in a short time, followed by a prolonged dry period.
Local Variations and Microclimates
While the official average provides a useful city-wide benchmark, San Francisco is famous for its localized climate differences, often referred to as microclimates. The distribution of rainfall is not uniform across the city limits, due primarily to topography and proximity to the Pacific Ocean.
The hills running through the city, such as Twin Peaks and Mount Sutro, create an orographic effect on incoming weather systems. As moist air is forced upward by the terrain, it cools and condenses, resulting in higher precipitation totals on the western slopes.
This effect means that neighborhoods closer to the ocean and situated on higher ground often receive several more inches of rain than the official downtown measurement suggests. Conversely, the eastern, bay-facing areas of the city can experience a slight rain shadow effect, recording lower annual totals.
The frequent summer fog delivers moisture, but this condensation is typically not measured as official liquid precipitation, even though it contributes to the local ecology.