Mount Tamalpais is the tallest peak in Marin County, rising to about 2,570 feet just north of the Golden Gate. Its weather is some of the most changeable in the Bay Area. The mountain stands squarely in the path of the summer marine layer, catches more rain than most of the region. It also packs several distinct microclimates into a single hike. On a summer day the summit can be sunny while the ocean-facing slopes sit in thick fog. The sheltered eastern side can be 20 degrees warmer than the ridges facing the coast. Knowing how the mountain works helps you decide what to bring and where to hike.
The Fog Machine: Western Slopes and the Marine Layer
In summer, the wind off the ocean pushes the marine layer straight into the mountain's western and southern faces. As the damp air meets the slopes, it is forced upward, and the fog thickens against the hillsides. The slopes that face the ocean, from Bolinas Ridge down toward Stinson Beach, are some of the foggiest ground in the Bay Area.
On that side, the Matt Davis and Steep Ravine trails can stay buried in fog in the low 50s while the summit sits in sunshine. The air feels damp, the trees drip and views can shrink to a few dozen feet. A hike that starts in sun can drop into that gray world within a few hundred feet of descent.
That fog is also what keeps the lower forests alive through the dry summer. As fog drifts through the redwoods and Douglas firs, droplets collect on the needles and fall to the ground. This fog drip waters the forest floor during months with no rain. Muir Woods, in a canyon on the mountain's southwestern side, is the best-known example of a forest that depends on it.
Summit Weather: Above the Marine Layer
On many summer mornings, the summit rises above the top of the fog. The marine layer is often 1,000 to 2,000 feet deep, so the East Peak, at about 2,570 feet, frequently pokes out into clear air. From the top, the fog can look like a flat white sea stretching to the horizon, with only the higher hills showing through. Those "sea of fog" views draw photographers to the summit at sunrise.
What happens next depends on how strong the marine push is that day. On days with a weak push, the fog burns off from the edges by late morning. The summit then opens up to views of the ocean, the bay and the city. When the push is strong, the fog deepens instead. It can climb past the summit by late morning and hide the whole mountain in cloud. If you want the view, go early, because the fog tends to rise or spread as the day goes on.
Winter brings a different kind of summit weather, with cold storms that can drop the temperature on top into the 30s. The exposed ridges also catch strong south winds ahead of each front. Every few years, a cold storm leaves a dusting of snow on the upper slopes, as happened in February 2019 and February 2023. Those mornings draw crowds, so the summit road can fill quickly.
Rainfall: The Wettest Mountain in the Bay Area
Mount Tamalpais is one of the wettest places in the Bay Area. The upper slopes average about 54 inches of rain a year, more than twice the roughly 23 inches that fall on downtown San Francisco. Kentfield, a town at the mountain's eastern base, averages about 43 inches.
The mountain's shape and position explain the extra rain. Winter storms usually arrive from the south and southwest, and the moist air has to rise to get over the mountain. As it rises, it cools, and more of its water condenses into rain. That process, called orographic lifting, drops extra rain on the slopes that face the storm and on the valleys just beyond them.
In wet winters, the mountain's creeks run high and waterfalls appear on slopes that are dry in summer. The Cataract Trail on the north side, above Alpine Lake, passes a chain of falls that are among the best in the Bay Area after heavy rain. That same runoff fills the reservoirs of the Marin Municipal Water District, which supplies much of the county. The mountain's rain is therefore a major part of Marin's water supply as well as its scenery.
Best Conditions for Hiking Mount Tamalpais
The best time to go depends on what you want to see. For long views from the summit, September and October usually offer the clearest days, because the fog weakens and the air is often dry. Spring brings green hills, wildflowers and full creeks, though some trails are muddy. Summer is the season for looking down on the fog from the top, but anyone hiking the western slopes should expect cold, wet air.
Whatever the season, bring an extra layer to Mount Tamalpais. The difference between a sunny trailhead on the east side and a windy, foggy ridge on the west can easily reach 15 to 20 degrees. A hiker who leaves Mill Valley at 72°F in a T-shirt can be shivering within minutes on the ridge above Stinson Beach. Because the mountain's weather changes so much from one side to the other, the regional forecast is only a starting point.
Fall brings its own hazard when dry offshore winds blow from the northeast. The mountain can then turn warm, breezy and very dry, even in late October. On those days fire danger runs high, and the water district and state park sometimes close trails or fire roads. Check for Red Flag Warnings before a fall hike, and avoid anything that could spark a fire.
After big winter storms, check for trail closures before you go. Heavy rain can bring down trees and set off small slides, and the narrow roads to the summit can close during storms. Between storms, though, the mountain is at its greenest, with clear air and creeks running full.
