Thick fog covering the Golden Gate Bridge

Bay Area Microclimates

Miriam Aczel

Mark Twain once said that “If you don’t like the weather in New England, wait a few minutes.” But in San Francisco, “If you don’t like the weather, walk a few miles.”

San Francisco is famous for its microclimates–local atmospheric conditions (e.g., rain, wind, sun, fog) that differ from those in surrounding areas. For example, you can start walking in an area on the Pacific Ocean side of San Francisco  and experience intense fog, climb up a hill to bask in brilliant sunshine, then face bone-rattling winds sweeping through high-rise skyscrapers. In fact, the temperature can differ by 10 degrees or more from one side of the city to the other, and these variations can become even more extreme in the summer!

Oakland: the best in the Bay

Did you know: Oakland supposedly has the best climate in the Bay Area! Though sunnier than San Francisco, the city has its own share of microclimates: while East Oakland tends to be drier and the Oakland Hills greener, areas near the San Francisco Bay such as Alameda and Emeryville are cooler and foggier. 

The extreme topographic variations in the Bay Area make it a good place to study how the uneven heating of Earth—due to changes in topography–leads to such different climatic patterns. California’s coastal mountain ranges are divided into a northern and southern portion by the San Francisco Bay; and due to the rain shadow effect—which causes a dry area on the side of a mountain opposite to the wind–the coastal side of the mountains is generally more humid and cooler, while the inland side is hotter and dryer. 

Urban microclimates

Human activity affects microclimates as well. Dense urban areas can have heat islands, where darker surfaces likes roads and buildings absorb more heat and trap it between buildings, creating urban hotspots that experience higher temperatures than surrounding rural areas. Factories and cars produce pollution as well, creating a pollution dome that traps the sun’s radiation and retains more heat. We can combat the urban heat island effect by using lighter, reflective surfaces on buildings, planting urban forests, and replacing darker rooftops with rooftop gardens.

Karl the Fog

The Bay Area’s unique microclimates shaped by differences in local geology and landscape. First, there is the cold Pacific Ocean and the Golden Gate Bridge—one of the few openings to the ocean on the coast. The Pacific is cold due to the California Current, which stretches southward from the Pacific Northwest and brings cold water from Alaska, and upwelling, where cold, deep water is brought to the surface of the ocean. Living in a neighborhood next to this cold body of water means you’ll feel the effects of the ocean, from weather patterns to winds! 

When the land mass heats up and meets the cold ocean air, the Bay Area’s famous fog, Karl, is created. Inspired by Karl the Giant in the 2003 film, Big Fish, the name really stuck—@KarltheFog has a Twitter following of nearly 350,000 and was once used as a clue on the game show, Jeopardy!

Are we losing ‘Karl’?

According to UC Berkeley plant ecologist Todd Dawson, fog from the California coast is an important source of water and nutrients for native plants like redwood trees. Fog also increases atmospheric moisture, reducing the risk of wildfires. However, the Bay Area’s microclimates are changing—a recent NBC Bay Area report showed that summer fog levels along the coast have been dropping due to rising temperatures. Over the last century, Bay Area residents have experienced a roughly 35% reduction in fog—or about three to four hours less of it per day. Having less fog can reduce regional humidity and increase wildfire risk, threatening the health, wellbeing, and safety of humans and wildlife alike. 

Microclimates affect everything, from farming and ecological management to urban planning. Planning strategies such as urban reforestation, rooftop gardens, and using reflective surfaces for construction materials, can help reduce temperature increases and play a role in protecting some of the area’s most iconic features, in addition to improving urban quality of life and comfort.

Karl the Fog drifts through the Golden Gate Bridge. Image credit: @KarltheFog

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