San Francisco is having one of its top-20-warmest years on record, but the city couldn't keep a late fall chill away over the weekend. The City by the Bay broke a record on Sunday when its high only reached 50 degrees.
"Tule fog" was the main contributor to the record cold day on Nov. 30. It's a type of fog that typically forms during the night and early morning hours from late fall through the beginning of winter. It is a type of radiation fog that occurs when longwave energy from the Earth escapes, cooling the air near the surface enough for low clouds and fog to form.
"It's called a Tule fog, but that's just a local reference to the phenomena that sets up in general, usually in the early winter," explained Roger Gass, the lead forecaster for the Bay Area National Weather Service Office, to SFGATE.
The fog acted like a blanket covering the sun, keeping the temperature around 10 degrees below average to set San Francisco up for a record-cold day. Sunday was the sixth straight day the city failed to climb above the 50s. The city's average high for the end of November is 60 degrees. The high of 50 on Sunday broke a record that had stood since 1922.
San Francisco had its 26th-wettest October on record, and November's rainfall of 2.52 inches was near normal. All that precipitation helped to saturate the atmosphere near the surface, "trapping moisture in the lower levels, resulting in this fog," added Gass.
The same conditions conducive to fog forming in the Bay Area can also lead to pollution being trapped near the surface. The Bay Area District had issued a "Spare the Air alert" at the end of last week, just a few days prior to Sunday.
"Smoke from increased wood burning combined with light winds and cold overnight temperatures is expected to cause elevated pollution levels," the alert warned. "High pressure over Northern California will act like a lid, trapping smoke at ground level. Offshore winds may also transport air pollution from the Central Valley into the Bay Area."
Winds have since picked up in San Francisco, dispersing the fog. Sunshine expected over the next several days is expected to push highs back into the 60s through the weekend. The concern now shifts to a coastal flood advisory for the Bay Area, in effect until 3 p.m. local time Sunday, and a beach hazards statement that remains in effect through Wednesday evening.
"Breaking waves 10 to 14 feet, with long lulls of 10 to 20 minutes or more between largest sets," cautions the National Weather Service. "For the Coastal Flood Advisory, up to 1.4 ft of inundation above ground level is possible in low lying areas near shorelines and tidal waterways. Dangerous conditions are forecast along the shoreline."
Despite San Francisco's chilly weekend, the city is having one of its warmest years on record as our planet continues to overheat. The first 10 months of this year represented the 16th-warmest January-through-October period on record for the city. The 24-month period from November 2023 through this past October was the city's fifth-warmest such period on record.
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