We’ve all noticed it. Whether we’re having a barbecue in a friend’s back patio, sunbathing in Dolores Park, or waiting in line for ice cream, we’ve all seen days where there seems to be a perfect circle of blue sky hovering over the Mission – but only over the Mission.
We have the famous San Francisco “microclimates” to blame. But what does that mean exactly, and how do they form?
The Department of Public Works lists three main microclimates in San Francisco, the Fog Belt that encompasses most of the city’s western neighborhoods, the Transition Zone which includes neighborhoods in the middle such as the Haight and the Marina, and the Sunbelt, which includes the Mission and other eastern neighborhoods.
The causes of this climatological complexity where the weather can change from block to block are pretty elemental: hot air, cold air, and hills.
“The short story is cold water to the West, warm valley to the East, and big hills blocking the fog,” says Paul Doherty, a senior staff scientist at the Exploratorium, which recently opened its new location at Pier 15 on the Embarcadero, near the Ferry Building, a Sunbelt locale.
“San Francisco is city of hills by the bay,” says Doherty. “But really it’s more a city of hills by the Pacific Ocean that causes all the microclimates.”
San Francisco weather begins in the “zone of winds” over the Pacific Ocean, a large wind system that circulates air and wind from Japan to the Arctic and then to the California coast. This wind pushes water south along California’s coastline.
That same ocean water passing along the coast is actually pushed away from the shore by something called the Coriolis effect - the tendency of an object on a spinning plane to deflect from moving on a straight line. Imagine a marble rolling on a spinning disk. It won’t move in a straight line and neither will the ocean water moving south along the coastline.
Instead, the Coriolis effect pushes the surface water away from the coast and out to sea. Cold deep ocean water rises to the surface to fill the space created by the displaced water, which makes for a very chilly coastal tidal zone. The cold water also makes for a nice, thick fog, which forms when the air above the water cools and condenses.
This cold ocean water is a substantial part of the equation that creates San Francisco’s notorious meteorological variations, but to make microclimates you need a few more ingredients.
To the East, hot air in the Central Valley adds another crucial ingredient. The difference in temperature between the cold coast and the hot Central Valley creates a wind that moves from the cold coast toward the hot east. As it travels, it pushes the ocean fog over San Francisco, enveloping the majority of the city’s western neighborhoods in low lying clouds.
“The Mission District has Twin Peaks to defend it,” says Doherty.
At more than 900 feet tall, Twin Peaks blocks the low lying fog. True, some manages to climb over the hill, but physics generally prevents it from traveling down the other side to cover the Mission. As an added bonus of wind protection, the Mission has Potrero Hill blocking breezes from the East and Bernal Hill offering shelter from the South.
“You can see it coming down like waterfall, but as it falls down it vanishes,” says Doherty of the fog’s climb over Twin Peaks. “As the air moves down hill it gets compressed, heats up, and evaporates.”
When the sun begins to set, the air cools and the fog stays cool enough at night to cascade down and cover the Mission.
In the fall, more of San Francisco gets to enjoy the Mission’s sunny weather because the Central Valley cools, Doherty says. Without the hot air rising in the East, there’s less wind to pull the fog across the city.
This is part of series attempting to address some of the Mission’s more scientific mysteries. You can read more here.