At the simplest level, fog is caused by global weather patterns and local hot air rising in the Central Valley pulling in cool ocean air laden with moisture. But the real story is much more interesting.

The story begins with the weather pattern out over the Pacific Ocean. The prevailing winds of the North Pacific High circle clockwise, owing to the Earth's rotation. This effects both the air mass and the ocean surface. As the moist, cool ocean air merges with air masses over the California land mass, it gives birth to a "ballet of fantastic flowing forms of aerial moisture" (SF Chronicle, June 12, 2000).

Look at the photograph of California at left, taken from space. The clear skies are caused by a high pressure area off the coast called the North Pacific High, which is a massive, semi-permanent high-pressure system that occupies most of the Pacific Ocean north of the equator. The photo also shows California's 500-mile-long Central Valley. The sun beats down on the Valley, heating the air to temperatures of 100 degress and more. This creates an onshore flow of cool, moist air.

On the right, air from out over the ocean north of California has run into cold water upwelling along the coast, which was caused by the Earth's rotation moving the warm surface water offshore. The combination of cool moist air and cold ocean water has created fog. The waves in the air mass are clearly seen. The mass flows along the coast and into the low parts of the coastal mountains: the river valley of the Russian River and the Golden Gate. You may have seen the ballet of fog as it flows over the Marin Headlands and crosses the Golden Gate Bridge. If you looked closely, you might have been able to see the waveforms of the fog. You certainly can see if it you are flying out of or into San Francisco Airport. (For a more complete analysis of the complex forces that cause our celebrated fog, see The Fog is Clearing.)

The main area of Camp Cazadero is an example of the first type, the alluvial-flat forest. It is characterized by deep soil that has been deposited by the river over geologic ages. This form of redwood forest provides the ideal growing conditions for redwood trees, and all of the groves of massive coast redwood grow in alluvial-flats. Daily temperatures are mild and seasonal changes are minimal, summer fogs are persistent, soils are deep, well-drained, and nutrient-rich, soil moisture is plentiful, even at the end of California's Mediterranean climate summer (characterized by no rain), and fires are less common and generally of low intensity. The best examples of the alluvial-flat redwood forest are those along the Smith, Klamath, and Eel rivers, but we also have some in the deeper canyons along the San Lorenzo and Big Sur rivers. It is not surprising that all of our famous coast redwood parks are in these areas (see this website for more information about parks: California Recreation). Big Basin State Park, the world's first redwood park (1901), is along the San Lorenzo river.

Notice in the photograph at the right that there are essentially no trees other than redwoods, and that these are all rather tall, with little in the way of understory. This is a classic alluvial-flat redwood forest.