One of the Bay Area's surprising weather facts is that Berkeley and the hills above Oakland get more rain than many places closer to the ocean. Berkeley averages about 26 inches a year, compared with about 23 in San Francisco, and the ridges behind Oakland get 27 to 29. Downtown Oakland, at the foot of the hills, gets about the same as San Francisco. Meanwhile, Palo Alto, on the bay side of the Peninsula, averages about 15 inches, and Mountain View about 16. The explanation comes down to the direction storms arrive from and the hills they have to cross along the way.
How Winter Storms Actually Enter the Bay Area
Most of the Bay Area's rain comes from Pacific storms that arrive from the west and southwest. Ahead of each cold front, moist winds blow in from the south and southwest, often carrying a plume of tropical moisture called an atmospheric river. The heaviest rain usually falls where those winds run into hills and are forced to rise.
The Peninsula has a large hill standing in that path. The Santa Cruz Mountains run along its ocean side, rising to about 2,000 feet behind the coast and higher farther south. Storm winds coming off the Pacific hit those slopes first. As the air rises, it cools and drops much of its moisture on the ocean-facing side. That is why the hills above Woodside and around La Honda are so wet.
By the time that air sinks down the far side toward the bay, it has lost much of its moisture and warms as it descends. That leaves bay-side cities such as Palo Alto, Menlo Park and Mountain View in a partial rain shadow, a drier zone downwind of the mountains.
Why the East Bay Hills Squeeze Out More Rain
On the far side of the bay, the hills rise again. The Berkeley and Oakland hills climb to about 1,900 feet just east of the cities, facing west and southwest across the water. When storm winds cross the bay and run into them, the air is forced up a second time. That orographic lift cools the air and wrings out more rain on the slopes facing the storm.
The rain falls hardest on the slopes themselves. That is why the ridges above Oakland get 27 to 29 inches a year while downtown, on the flats a few miles west, gets about 23. Berkeley sits close to the base of the hills, so it also catches part of that extra rain. Beyond the ridge, towns such as Orinda still get plenty, but farther east, in the Diablo Valley, the totals drop again as the air sinks and dries.
The flat bay-side Peninsula has nothing like that wall of hills right behind it. Its cities sit on low ground between the bay and the drier, downwind side of the Santa Cruz Mountains. With no second lift nearby, they collect much less rain from the same storms.
The Golden Gate Gap and the Rain Shadow
Berkeley has one more advantage, which is its position across the bay from the Golden Gate. The Gate is the only sea-level opening in the coastal hills for many miles. Moist ocean air that streams through it reaches the bay without first climbing over any mountains, so it arrives with its moisture largely intact.
Storm winds from the west and southwest carry that air across the bay toward Berkeley, Albany and Richmond. When it reaches the East Bay hills, it is still full of water, and the lift squeezes out generous rain. Cities to the south, by contrast, lie behind the Santa Cruz Mountains and get air that has already been wrung out.
The pattern is not the same in every storm. A storm with winds from a slightly different direction, or one with heavy showers drifting in after the front, can spread rain more evenly. Over many years, though, the totals add up to a clear difference between the two sides of the bay.
Why the South Bay Is Drier Than the North Bay
Across the region, rainfall generally drops from the north and west toward the south and east. The wettest spots are in the hills of Marin and the North Bay, such as the slopes of Mount Tamalpais, which face the incoming storms head-on. San Francisco and the East Bay fall in the middle. The bay-side Peninsula and the South Bay are drier.
San Jose averages only about 15 to 16 inches a year, less than most of the Bay Area. It sits on the valley floor at the southern end of the bay, deep in the rain shadow of the Santa Cruz Mountains to its west. The hills around the valley catch more rain, but the flatlands stay relatively dry.
The inland valleys east of the hills are drier still. Livermore gets about 15 inches, and Antioch, farther east toward the Delta, only about 13. Air that crosses both the coastal mountains and the East Bay hills has little moisture left by the time it sinks into those valleys.
Deep Dive: Why Doesn't the Rain Stop Exactly at the Ridge?
Rain formed over a ridge does not fall straight down onto it. The wind carries drops sideways as they fall, so some of the rain lands beyond the crest. Meteorologists call this spillover, and it softens the edge of the rain shadow just past the hills.
How Far Do Raindrops Drift?
Raindrops fall at speeds set by their size. Classic measurements by Ross Gunn and Gilbert Kinzer in 1949 found that a 2-millimeter drop falls at about 6.5 meters per second. A 1-millimeter drop falls at about 4 meters per second, and a drizzle drop of 0.2 millimeters at less than 1.
By our calculation, a 1-millimeter drop falling from 6,500 feet in a 33-mph wind drifts nearly 5 miles before it lands. A drizzle drop from the same height could drift more than 25 miles. Larger drops land close to the ridge, while the finest drops and snowflakes are carried well beyond it.
That helps explain why Orinda, just over the Berkeley and Oakland hills, still gets plenty of rain. Much of what falls there formed over the western slopes and drifted across the crest. Rain gauges just east of a ridge can record totals close to those on the crest itself.
Why Does the Rain Shadow Deepen Farther Downwind?
Beyond the spillover zone, the air begins to sink. Sinking air warms by compression, and warmer air can hold more water vapor. Cloud droplets and small raindrops falling into it start to evaporate.
Once the clouds have evaporated, the sinking air warms by about 5.5°F for every 1,000 feet it descends. Its relative humidity falls quickly, and the remaining drops shrink as they fall. Light rain that would reach the ground on the windward side can vanish before it lands in the lee.
Together, spillover and sinking set the shape of the rain shadow. Totals stay high for a few miles past the crest, then drop off as the air keeps sinking toward the Diablo Valley. That is why Walnut Creek and Concord, farther from the ridge, are noticeably drier than Orinda.
What These Differences Mean Day to Day
These differences shape the landscape. The East Bay hills stay greener and lusher through the rainy season than the flat, bay-side parts of the Peninsula. Creeks in parks such as Tilden, Redwood and Anthony Chabot run fuller in winter, and some keep flowing well into spring.
For hikers, the extra rain means East Bay hill trails often stay muddy for a day or two longer after a big storm. Trails on the drier, flatter parts of the Peninsula and South Bay dry out sooner. The reward is greener hills, fuller creeks and the occasional seasonal waterfall in winter and spring.
The pattern also matters when a big storm is coming. Forecasts often call for the highest totals in the hills of Marin, the Santa Cruz Mountains and the East Bay. San Jose and the inland valleys usually get less. Knowing where you live within that pattern helps you judge how much rain, and how much flooding risk, a given storm is likely to bring.
