The coldest temperature ever recorded in San Francisco is 27°F, set downtown on December 11, 1932. Downtown San Francisco has not dropped to freezing since December 1990. The Bay Area gets much colder inland: Livermore has recorded lows of 18°F, most recently on December 22, 1990.
Bay Area cold comes mostly from clear, calm nights rather than Arctic air. On those nights, heat radiates to space and cold air pools in low valleys. Valley floors can end up 10 to 15 degrees colder than the hills just above them.
What Is the Coldest Temperature Ever Recorded in San Francisco?
San Francisco's coldest temperature on record is 27°F, set downtown on December 11, 1932, in records that begin in 1874. The second-coldest reading is 28°F, on December 22, 1990, during a statewide freeze. San Francisco International Airport, on the bay shore, has dropped as low as 24°F, on December 9, 1972.
The ocean keeps San Francisco relatively warm in winter, because the coastal water rarely drops much below 50°F. Cold air crossing that water warms before it reaches the city, and January nights downtown average about 47°F.
How Often Does San Francisco Drop Below Freezing?
Downtown San Francisco almost never drops to freezing. The station has recorded 32°F or colder on only 8 days since 1950, and none since December 24, 1990. Those freezes came in 1962, 1972 and 1989, and during the December 1990 cold wave, which brought four freezing nights in a row.
Even in a cold spell, a freeze in the city rarely lasts past sunrise. Snow is rarer still, as our guide to whether it snows in San Francisco explains.
What Is the Coldest Temperature Ever Recorded in the Bay Area?
The inland valleys hold the Bay Area's cold records. Livermore has recorded 18°F three times: January 5, 1961, December 9, 1972 and December 22, 1990. San Jose's record low is 22°F, set in January 1917. Hard freezes in the Livermore Valley, the Napa Valley floor, the Santa Clara Valley around Morgan Hill and Gilroy, and the interior Sonoma County valleys reach the mid-20s.
The coldest spots are flat valley floors far from the bay and the marine layer, under clear skies on calm winter nights. Winemakers in Napa and Sonoma track these lows closely, because a spring frost during bud break can destroy a year's crop in one night.
Why Are Bay Area Valleys So Cold at Night?
Bay Area valleys are cold at night because of radiation cooling. On clear, calm winter nights, the ground radiates heat to space and chills the air touching it. Cold air is denser than warm air, so it drains downhill and pools on valley floors under a temperature inversion, a layer of warmer air above. When that pooled air cools to its dew point, the same setup makes tule fog. At or below 32°F, that fog becomes freezing fog, which coats roads, bridges and trees with ice.
Bay Area cold snaps are shallow, unlike the Arctic outbreaks that freeze Chicago or Denver. The cold pool is often only a few hundred feet deep, and the hillsides above it can be 10 to 15 degrees warmer. That is why farmers protect crops with wind machines that mix warmer air down to the ground. Wind or clouds end the cold quickly by stopping the heat loss.
