Downtown San Francisco was treated to a rare meteorological sight Wednesday morning: dense fog that rolled in from what feels like the wrong direction. Moving from east to west, it enveloped South of Market and much of the Mission within minutes. With the western half of the city relatively sunny, it was the opposite of a typical July day.
The National Service issued a “dense fog advisory” for much of the Bay Area until 9 a.m., then extended it to 11 a.m. and expanded it to cover San Francisco. Earlier in the day, SFO was reporting significant delays. But by 10:15 a.m., the fog had largely melted away.
The phenomenon is known as tule fog. It’s different from the marine layer that often blankets the city in summer months, and it can be hazardous. Drivers frequently encounter visibility of less than a quarter-mile, and changes in conditions can be abrupt.
Named for the tule grass that grows in California wetlands, the fog is common in the Central Valley but rare along the coast. It occurs after periods of rain, when humidity is high and nights are long; this allows the ground to cool for longer periods, forming thick fog.
Did you catch this morning’s Tule fog invasion? Send your pictures to tips@sfstandard.com, and we will post ’em.