Annual Average Rainy Days

San Francisco receives an average of about 60 days of rain per year. This is relatively high even within California, with most rain occurring in the winter and early spring. The summer months are generally dry, but rain can occasionally occur due to phenomena like the Santa Barbara effect or lake effect.

Monthly Average Rainfall

The monthly average rainfall in San Francisco varies significantly depending on the season and oceanic influences. The average rainfall for each month is as follows:

San Francisco

  • January: about 4.5 inches (about 115mm)
  • February: about 4.0 inches (about 102mm)
  • March: about 3.3 inches (about 84mm)
  • April: about 2.1 inches (about 53mm)
  • May: about 1.2 inches (about 30mm)
  • June: about 0.3 inches (about 8mm)
  • July: about 0.1 inches (about 3mm)
  • August: about 0.1 inches (about 3mm)
  • September: about 0.4 inches (about 10mm)
  • October: about 1.0 inches (about 25mm)
  • November: about 3.0 inches (about 76mm)
  • December: about 4.3 inches (about 109mm)

Main Features:

  • Winter (December to February): The period with the most rain, where continuous rainfall and frequent thunderstorms can occur.
  • Spring (March to May): Rain gradually decreases, leading to drier weather. However, strong rain can still occur in March.
  • Summer (June to August): A very dry period with extremely low average rainfall. However, rain can occasionally occur due to the Santa Barbara effect or lake effect.
  • Autumn (September to November): Rain increases again, especially in October and November. The influence of the monsoon can lead to a slight increase in rainfall.

Record Rainfall: The highest recorded rainfall in San Francisco occurred during a heavy rainstorm in January 1998, with 14.71 inches (about 374mm) of rain. This rainfall caused severe flooding and inundation in the area due to the combination of heavy rain and the El Niño phenomenon.

The climate of San Francisco features characteristics of a maritime climate, with an average of 60 rainy days per year and monthly average rainfall varying significantly due to seasonal and oceanic influences. Although the rainfall is relatively low, exceptional downpours can greatly impact transportation and daily life. Understanding these climatic characteristics and preparing countermeasures can greatly enhance the comfort and safety of living in San Francisco.