Hikers enjoy an autumn stroll on the Canyon View Trail in Sunol Wilderness.

      Climate

      The East Bay has one of the best climates in the United States for year-round outdoor recreation: it is rarely too hot or too cold to go hiking somewhere here. When summer’s heat and humidity drive residents of other parts of the country to seek air conditioning or the beach, we can enjoy a stroll through cool, fog-shrouded groves of coast redwoods. And when the northern half of the United States is locked for months on end in winter’s icy grip, we can often go outdoors with nothing more than a sweater and a windbreaker, taking advantage of clear skies to climb a peak and gaze at the snow-capped Sierra.

      Instead of four seasons, the Bay Area has two: dry, lasting from May through October, and wet, generally from November through April. (Residents of San Francisco have a third season, fog, during the summer months, prompting Mark Twain’s famous statement that the coldest winter he ever spent was the summer he spent in San Francisco.) Time of year can have a dramatic effect on trail conditions and the character of a particular hike. You can broil on some routes during the summer, and find others nearly impassable because of mud in the winter. Most of the trips in this book are enjoyable during spring and fall. Check Appendix 1 for the best summer and winter trips.

      At the start of the dry season, the hills are green and decorated with blooming trees, shrubs, and wildflowers. But without rain, the hills gradually turn from green to brown, seasonal creeks dry up, and water levels in lakes and reservoirs fall. Skies are blue, but as spring gives way to summer, ocean breezes from the west and thermal low pressure over the Central Valley propel ocean fog over the western hills and through the Golden Gate into San Francisco Bay, where it often lingers for days on end, sometimes climbing up and spilling over the Berkeley Hills.

      With the coming of fall, wind patterns shift and the fog is pushed out to sea. This is a time of extreme fire danger, with plenty of dry fuel and warm, dry winds. It is also a time of intense beauty in the East Bay, when the leaves of bigleaf maple, western sycamore, poison oak, and California wild grape take on autumnal hues, and the grasses that blanket the hills are golden brown. As high pressure over the Eastern Pacific weakens, the way is clear for storms to move in from the Gulf of Alaska or the sub-tropics. When the rains finally arrive, the East Bay undergoes a magical transformation, turning from brown to green almost overnight. Creeks fill and swell, often overflowing their banks and spilling onto the trail. Even as the calendar says winter, our early blooming manzanitas announce the coming of spring with clusters of white or pink flowers.

      In addition to being influenced by time of year, conditions vary depending on where you are in relation to San Francisco Bay. The wind here generally blows from west to east, bringing cool, moist air inland from the Pacific Ocean. Starting with the Oakland and Berkeley hills and going east, each successive set of hills presents a further barrier to this marine air, making nearby valleys progressively hotter and drier in summer. So while Tilden Park in the Berkeley Hills might be comfortable in July, Mt. Diablo, farther east, would be unpleasantly hot. But the waters of the Bay also have a stabilizing effect on temperature, keeping areas near its shore cool in summer and relatively warm in winter. As you move east, away from the Bay, this effect lessens and temperature extremes increase. So in January, you might find it warmer in Berkeley than in, say, Concord.

      Although our climate—average conditions over the course of a year—is mild, our weather—daily atmospheric conditions—can be exciting. Wind is perhaps the most unpredictable condition, sometimes blowing ferociously on an otherwise perfect day, at other times disappearing as you make a slight change in elevation or orientation. Strong winds can turn a pleasant hike into an ordeal, and can even be hazardous, knocking down trees and power lines. But wind can be a bonus too, bringing relief on a hot day or clearing the air after a winter storm. You can use a weather radio, available at Radio Shack, outdoor stores, and other outlets, to receive broadcasts from the National Weather Service. You can also find up-to-the minute weather information on the Weather Channel or on the Internet at www.weather.com.