Rain does not fall evenly across the Bay Area. Within about 30 miles, average yearly totals range from around 14 inches to more than 50, and the hills explain most of that difference. When Pacific storms run into high ground, the hills squeeze extra rain out of the air, a process called orographic enhancement. That is why the Mount Tamalpais watershed in Marin gets about twice San Francisco's rainfall, and why the Berkeley hills get less than Tamalpais's higher slopes. It is also why some of the region's lushest forests grow without any irrigation at all.
How Orographic Rain Forms
When moist air reaches a hill or mountain, it is forced upward. Rising air expands and cools, by about 5.5°F for every 1,000 feet it climbs until it becomes saturated. Once it cools to its dew point, its water vapor condenses into cloud droplets. If the lifting continues, the droplets grow large enough to fall as rain or snow. The windward side of the mountain, which faces the incoming air, gets the extra rain. The leeward side, where the air sinks and warms, stays drier, an area known as a rain shadow. When stable, nearly saturated air is lifted gently, the same process can produce fog on the slope instead of rain, called upslope fog.
The Bay Area's terrain is well placed to wring extra rain from winter storms. The wettest storms usually arrive on west to southwest winds carrying moisture from the open Pacific. The Marin hills, the Santa Cruz Mountains, the East Bay hills and the Diablo Range all lie across that flow and force the air upward. The steeper and higher the terrain, the stronger the effect. Mount Tamalpais, at 2,574 feet, gets far more rain than the low East Bay hills, which in turn get more than the flat South Bay.
Bay Area Rainfall Extremes
San Francisco, on the fairly low tip of the Peninsula, averages about 23 inches of rain a year. The Marin watershed behind Mount Tamalpais and Bolinas Ridge gets 40 to 55 inches under the same storms, roughly double San Francisco's total. Kentfield, at the base of Mount Tamalpais, averages more than 40 inches, and the upper mountain gets more than 50.
The Santa Cruz Mountains show the same pattern on a larger scale. Ridge-top communities like La Honda and Skylonda average 40 to 50 inches a year, while San Jose, in the sheltered Santa Clara Valley, averages only about 14. Much of the range is only 2,000 to 3,000 feet high, with Loma Prieta topping 3,700 feet, but its western slopes face the incoming storms directly. That extra rain, along with summer fog, lets redwood forests grow on the ocean side of the range. Drier oak woodland and chaparral cover much of the inland side.
Fog Drip: The Invisible Rainfall
Coastal hills also collect water that rain gauges miss, called fog drip. When the marine layer flows over coastal ridges, fog droplets collect on leaves and needles and merge into drops large enough to fall. A dense coastal forest can capture the equivalent of several inches of extra rain in a season this way. Research on coastal redwoods has found that fog water makes up a significant share of the water they use in summer, when rain is almost absent. Fog drip is not a curiosity for these forests but an important part of their summer water supply.
Muir Woods gets almost no summer rain, yet the forest stays lush through August. Fog drip is part of the reason, as the tall redwoods comb moisture out of the marine layer that drifts up Redwood Creek from Muir Beach. The water falls drop by drop to the forest floor and keeps the soil around the roots damp. The trees do more than benefit from the fog, because their height and dense needles help create the moist conditions that sustain them. The same lift of ocean air up the coastal slopes that boosts winter rain also helps bring that fog to the trees.
Rain Shadows: The Dry Side
The leeward sides of Bay Area mountains get much less rain than the windward faces. The western slopes of the Berkeley hills get 25 to 30 inches a year. The Diablo Valley, in their rain shadow to the east, gets about 18 to 20. Livermore and the rest of the Tri-Valley sit behind several ridges and get only 14 to 16 inches. Orographic enhancement and its opposite, the rain shadow, explain why Bay Area rainfall maps look so uneven. The totals swing widely, but they follow the terrain closely.
Deep Dive: How Much Extra Rain Does a Hill Squeeze Out?
The rain a hillside gets depends mostly on how much moisture the wind carries straight into it. Researchers have measured that link directly on the coastal mountains north of the Bay Area. They have also found a second process in which clouds high above can multiply the rain from lower clouds caught on the slopes.
What Did Measurements on the Coast Show?
During the CALJET field experiment in the winter of 1997-98, NOAA scientists set up wind profilers, radars that measure wind at many heights, along the Sonoma County coast. One was at Bodega Bay, near the shore, with a rain gauge in the coastal mountains at Cazadero. Paul Neiman, Marty Ralph and colleagues compared the two in a 2002 paper in Monthly Weather Review.
Rain in the mountains tracked the speed of the wind blowing toward the slopes. The link was closest at about 1 kilometer up, near the height of the mountaintops. Upslope wind at that level explained up to half of the hour-to-hour changes in mountain rain rate. When a low-level jet, a band of fast wind just above the ground, was present, the mountains turned moisture into rain about 50 percent more efficiently.
By our calculation, picture an atmospheric river carrying 500 kilograms of water vapor across each meter of coastline every second. If the coastal mountains wring out a fifth of that over a 12-mile-wide strip, they get about 0.7 inches of rain an hour. That is a rough match for the heaviest rates seen in the Bay Area hills during strong storms.
What Is the Seeder-Feeder Effect?
Lifting air over a hill often creates a low cloud capping the slope. That cloud is full of tiny droplets, too small to fall as rain on their own. The Swedish meteorologist Tor Bergeron described what happens when rain or snow from a higher cloud falls through it.
The falling drops act as seeds. As they pass through the low cloud, they sweep up its tiny droplets and grow larger. Scientists call the upper cloud the seeder and the low hill cloud the feeder. Together they can produce far more rain on the slope than either cloud would alone.
The effect helps explain why rain gauges a few hundred feet apart in elevation can record very different totals in the same storm. It also helps explain why rain can change so sharply along a single slope, depending on where the feeder cloud sits.
