The Altamont Pass is a low gap in the hills east of Livermore, where Interstate 580 climbs out of the Bay Area and drops into the Central Valley. It is one of the windiest places in California, and the rolling grass hills on both sides of the freeway are covered with wind turbines. The wind comes from a simple contrast in temperature. On summer afternoons the Central Valley bakes while the Bay Area stays cool, and the cool air pours east through the gap toward the heat. That steady west wind is what drivers feel on I-580, and it is the reason a wind farm has operated on these hills since the early 1980s.
Why Altamont Is So Windy
On a summer day the Central Valley often heats past 100°F. Hot air is lighter than cool air, so the valley develops an area of low pressure called a thermal low. Air flows from higher pressure toward lower pressure, so the cool, dense air near the coast starts moving inland through every opening in the hills.
One of the main openings is the Altamont, which tops out at only about 1,000 feet, much lower than the ridges to the north and south. Cool air from the Livermore Valley squeezes through this saddle and speeds up as it goes, much as wind picks up when it funnels between two tall buildings. During a typical summer afternoon, the wind in the pass blows 20 to 30 mph, and gusts on strong days can reach 40 to 50 mph.
This is not the only gap in the hills that carries the flow. Farther north, the Carquinez Strait lets ocean air reach Sacramento and the Delta, where people call it the Delta breeze. Both gaps work the same way, and on hot days they pull cool air inland at the same time. That is why the Altamont and the Delta often turn windy on the same afternoons.
The wind is strongest from late spring through summer, when the gap between coastal and valley temperatures is largest. In July, San Francisco might sit near 65°F in the afternoon while Stockton reaches 100°F or more. A contrast that large drives a strong push of air through the pass. The wind usually builds through the afternoon and peaks in the late afternoon and evening, often blowing well into the night. In winter, when the coast and valley are closer in temperature, that daily push fades. The pass still gets strong winds then, but they come with passing storms rather than on a daily schedule. Winter brings other hazards to the pass as well. Tule fog from the Central Valley sometimes drifts up into the pass on cold mornings and cuts visibility on I-580. In the coldest storms, every few years, the highest hills also get a light dusting of snow.
The Altamont Driving Experience
Most passenger cars handle the pass without trouble, though drivers can feel the car being pushed sideways on the open stretches. Tall or light vehicles feel the push much more strongly. RVs, box trucks, empty big rigs, trailers and cars carrying kayaks or roof boxes catch the crosswind like a sail. On the windiest days, Caltrans uses its overhead message signs to warn drivers of high-profile vehicles about strong gusts.
If you drive a tall vehicle, slow down and keep both hands on the wheel through the pass. Watch for gaps between hills, where a gust can hit suddenly after a sheltered stretch. Leave extra room around other high-profile vehicles, which can drift out of their lanes. Dust and loose grass sometimes blow across the road too, especially in late summer when the hills are dry.
The time of day you cross makes a real difference. The pass is usually calmest in the morning, before the valley heats up, and windiest from mid-afternoon into the evening. If you are towing a trailer or driving an RV in summer, a morning crossing is often the easier choice. In winter, check the forecast for storm winds and morning tule fog before you go.
Driving west from the Central Valley on a hot summer day shows the whole pattern in a few miles. You can leave Tracy or Stockton at 100°F, climb into the pass with the wind hitting your car, and come down into Livermore noticeably cooler. Livermore is still warm by Bay Area standards, but the air keeps getting cooler as you continue west toward the bay. It is the same air you are driving into that streams east through the pass.
Wind Turbines and Bird Mortality
The Altamont Pass Wind Resource Area was one of the first large wind farms in the world. In the 1980s, developers packed the hills with about 5,000 small turbines. That made it one of the largest wind installations of its time. The machines were short and fast-spinning, and many stood on lattice towers made of crossed steel bars.
That early design turned out to be dangerous for birds. The surrounding grassland holds plenty of ground squirrels and other prey, so golden eagles, red-tailed hawks, American kestrels and burrowing owls hunt across the hills. Researchers found that the fast blades killed large numbers of these birds. Studies tracked by the U.S. Fish and Wildlife Service made Altamont a national example of the conflict between wind power and wildlife.
The same wind pattern makes the pass valuable for power. Because the wind is strongest on summer afternoons and evenings, Altamont turbines produce most of their energy in the warm months. That timing lines up well with the hours when California's demand for electricity climbs as people run air conditioning.
Since the mid-2000s, operators have been replacing the old turbines through a process called repowering. Far fewer, much larger turbines on smooth tubular towers now do the work of many small ones, and their blades turn more slowly. The number of bird deaths has dropped, though it has not reached zero, and wildlife groups continue to press for further changes. It remains a reference case wherever new wind farms are planned near raptor habitat.
