A Major US city breaks 103-year-old record due to unusual 'Tule fog' phenomenon: 'Forecast predicts dangerous conditions'

San Francisco, despite ranking among the top 20 warmest years on record, faced an unusually chilly late fall weekend. On Sunday, the city recorded a high of only 50 degrees, setting a new low-temperature record for the date.

The unusual cold was largely caused by Tule fog, a type of radiation fog that forms during the night and early morning from late fall to early winter. It occurs when heat radiates from the Earth's surface, cooling the air enough to generate low clouds and fog.

"Tule fog is a local term for a phenomenon that generally appears in early winter," explained Roger Gass, lead forecaster at the Bay Area National Weather Service, to SFGATE. The fog acted like a thick cover over the sun, keeping temperatures about 10 degrees below the seasonal average.

Sunday marked the sixth consecutive day San Francisco struggled to reach the 50s. Historically, the city's average high at the end of November is around 60 degrees. The 50-degree high set on Sunday surpassed a record that had stood since 1922.

Recent rainfall contributed to the fog. San Francisco recorded its 26th-wettest October, and November received 2.52 inches of rain, near normal. Gass noted that saturated air near the surface trapped moisture, enhancing fog formation.

These conditions can also trap pollution close to the ground. The Bay Area Air Quality Management District issued a Spare the Air alert last week, warning that smoke from increased wood burning, combined with light winds and cold nights, would elevate pollution levels. High pressure over Northern California prevented dispersion, while offshore winds transported additional pollution from the Central Valley.

Winds have since strengthened, dispersing the fog. Sunny conditions over the coming days are expected to return highs to the 60s through the weekend.

The focus now shifts to coastal risks. A Coastal Flood Advisory remains in effect until 3 p.m. Sunday, and a Beach Hazards Statement continues through Wednesday evening. The National Weather Service warns of breaking waves 10 to 14 feet high, with long lulls between sets, and potential inundation up to 1.4 feet in low-lying areas.

Despite the chilly weekend, San Francisco continues to experience one of its warmest years. January through October ranked as the 16th-warmest such period on record, while November 2023 through October 2025 represents the city's fifth-warmest 24-month span.

Author: Sophia Brooks