Why Does The Bay Area Have So Many Microclimates?

The Bay Area’s microclimates, which stem from the proximity to water and coastal terrain, can lead to both dramatic weather variations and views — like this one of Yerba Buena Island and San Francisco captured from the Berkeley Hills.  (Melinda Young Stuart/Flickr)

It bewilders Bay Area newcomers — at least, the ones who show up wearing sandals and sunglasses and quickly find themselves shopping for a sweatshirt. But as any local will attest, here you can enjoy clear skies and mild temperatures the same day you experience whipping winds and clammy fog, often just a few short miles apart. Microclimates are everywhere.

That got Bay Curious listener Scott wondering: “Bay area microclimates! Why are there so many? Where are they? How do they differ?”

We’ll get to all of that, as well as another listener question to do with weather, but first let’s define the word microclimate. Casually, it refers to the phenomenon of sometimes vastly different weather in what feels like adjacent zip codes, like how it’s routinely foggy in San Francisco’s Richmond District, while a few miles away the Mission enjoys sunshine.

But this usage of the term “microclimate” isn’t the most scientific, says Andrew Oliphant, who studies micrometeorology as a professor in the department of Geography and Environment at San Francisco State University.

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“When we talk about microclimates of the Bay Area, we’re actually a little bit beyond the traditional scales of micro,” Oliphant says. A proper microclimate might range from less than a city block up to about half a mile. So when denizens of the Bay Area bemoan its microclimates, “we’re really talking more neighborhood-to-neighborhood scale.”

And these many variations don’t lend themselves to neat lines on a map. They’re more like fine gradations, making it tough for experts to pinpoint how many microclimates there are in the Bay Area.

So why so much variation?

View of San Francisco from Cesar Chavez Park in Berkeley.
Because it's surrounded on three sides by water, San Francisco tends to enjoy relatively mild temperatures, but that benefit diminishes further inland, leading to disparate microclimates. (Daniel Potter/KQED)

There are a few reasons for our variable conditions, says Darrel Hess, who is the author of a physical geography textbook, as well as an instructor at City College of San Francisco.

“One is our location next to the water,” he says. San Francisco, with water to its north, east and west, rarely gets much hotter or cooler than the ocean. The further inland you get, the less of that benefit you enjoy.

“As you go over each ridge in the coast ranges, as you move away from the ocean, the weather and climate becomes increasingly continental — in other words, you have less ocean influence,” Hess says. That’s why a city like Livermore, some 20 miles east of the Bay, can get downright hot in summer and chilly in winter.

Terrain is also a factor, as when a summer fog seeps from the ocean toward lower elevations around the Bay Area.

“Almost always it comes in first right through the Golden Gate. That’s the only sea-level opening in the coast ranges,” Hess says.

And the same is true for rain. When a winter storm makes its way east from the ocean, mountains in the storm’s path are sure to get drenched, while the sheltered places behind them, not so much. The windward side of the Santa Cruz mountains might soak up several inches of rainfall while, further east, San Jose gets just a fraction of that.

“We say San Jose’s in the rain shadow,” Hess says.

Hot city, weird weather?

Our second question on weather came from Yvine, who asked, “Does the urban heat island have any influence on San Francisco Bay's weather process?”

Urban heat islands are sprawling cityscapes with lots of asphalt and not much greenery. They get and stay hotter on bright days, because buildings and parking lots tend to hold more heat from the sun than vegetation. And they can indeed affect the weather, at least in some places.

For instance, in Houston, Texas, a thunderstorm might be right on the edge of happening if the air gets just a little hotter. If the only place that’s quite hot enough is an urban heat island, a sudden storm could pop up just in that spot.

But does this happen in the Bay Area?

It turns out, San Francisco isn’t a great example of an urban heat island. Being surrounded on three sides by water limits the requisite sprawl. But Oliphant says you can feel the difference it makes in a place like Golden Gate Park, where some days it might be seven degrees cooler than the surrounding neighborhoods.

And the effect may be more pronounced in other parts of the Bay Area, like Oakland and the South Bay, says Daniel Swain, a climate scientist with joint affiliations with UCLA, the National Center for Atmospheric Research and The Nature Conservancy of California.

Swain doubts heat islands affect the weather here much, but as someone who models the complex atmosphere, he says even small nudges can have surprising impacts, "so I would never say never."

Wind tunnels and fog tendrils

San Francisco’s built environment may also affect the weather, says Swain. He points to how wind is channeled between the growing number of tall buildings downtown.

“You have wind blowing through these man-made canyons and tunnels, and then that causes the air near the surface to mix a lot with the air from above the surface,” he says.

That mixing has decent potential to influence the weather, at least slightly. As a thick fog creeps from the Financial District toward the Bay, it might just be enough to change which way a few tendrils of fog are headed next.