San Francisco is famous for its unique climate, characterized by persistent summer fog, mild temperatures year-round, and a lack of extreme weather events. The city’s location within a Mediterranean climate zone, defined by wet, mild winters and dry summers, makes the deep freeze required for snow highly unusual.
The Definitive Answer and Rarity
The short answer is that yes, it has snowed in San Francisco, but the event is so rare that most current residents have never witnessed measurable accumulation. Snowfall in the downtown area is considered a once-in-a-generation occurrence, often appearing as brief, non-accumulating flurries. Since record-keeping began, the city’s official observation sites have documented only about ten instances of measurable snow. This extremely low frequency means that when snow does fall, it is typically a light dusting that melts quickly upon contact with the ground.
Historical Instances of Measurable Snow
The most significant snowfall in San Francisco’s recorded history occurred on February 5, 1887, setting the all-time record for accumulation. The downtown weather bureau measured 3.7 inches of snow, an amount more than double any other recorded event. Higher elevations in the western parts of the city saw depths up to seven inches.
The last time measurable snow settled on the city was on February 5, 1976, nearly a century after the record storm. This event brought approximately one inch of snow to the lowest parts of the city, while higher peaks, such as Twin Peaks, saw accumulations of up to five inches. Earlier heavy snows also fell on December 30, 1856 (2.5 inches), and on New Year’s Eve 1882 (3.5 inches downtown).
Geographic and Climate Factors That Prevent Snow
The primary reason for the absence of snow is San Francisco’s geography and the moderating influence of the Pacific Ocean. The city sits within a maritime climate where the ocean water temperature remains relatively consistent. This large, cold body of water keeps winter temperatures from dipping below the freezing point necessary for snow to form and accumulate at sea level.
Prevailing winds generally flow onshore, pulling mild, moist air from the Pacific over the city, which prevents the sustained intrusion of continental cold air masses. Higher points like Twin Peaks (over 900 feet) can occasionally see a light dusting when lower areas only receive rain, illustrating the effect of minor elevation changes.
Specific Meteorological Conditions Required
For snow to fall in San Francisco, a unique combination of atmospheric anomalies must overcome the city’s stable climate. Specifically, a very cold air mass is required, usually originating from the Gulf of Alaska or the Arctic, which must penetrate far south into the region. This must coincide with a weather system bringing precipitation.
A crucial factor is the freezing level, the altitude where the temperature drops to 0°C, which must fall dramatically. Snow requires the freezing level to drop significantly, typically below 1,000 feet, for the snowflakes to survive the descent to sea level without melting into rain. The 1976 event, for example, was supported by an unusually cold 500 millibar low-pressure system.