One weekend evening last month, after an exhausting day of strategic planning for twenty people, several of us made a trip up a nearby peak. Descending in the moonlight, our path would occasionally give fantastic views of local valleys and nearby towns with twinkling lights. Some of the western peaks had fog pushing up and over them, or pushing its moonlit blue shroud around their flanks and up into the valleys. It was warm near the summit; estimating by the tree crickets and our experience, nearly seventy degrees and dry as August can be. Somewhere near the base of the hill, the air suddenly turned cool and moist, even though no clouds were around. It was the first breath of approaching fog.

Fog is an amazing gift to our region and valley. Many plants in the coastal zone are so linked to fog that they can barely live without it. Much of our local agriculture and economy are also linked to the forces that make fog. It is an interesting story, the formation of our cool afternoon breezes and evening fog. It is, as most things in life, only by the tremendous fortune of countless circumstances working together that we have this famous climate.

We live on the western edge of a continent, at a latitude where air that has risen in warmer parts of the world happens to fall back to earthÐcool, heavy and dryÐin what is known as the "Pacific High". This mass of subsiding or falling air creates a high pressure zone off our coast several hundred miles. It remains stable for months in the late spring and summer, and it keeps our area mostly free of rain as it pushes away more unstable (stormy) air. As the air in a high pressure region falls, it moves outward along the surface of the earth, and because the Earth is spinning, the air turns in a clockwise direction here. This then, creates a prevailing wind that blows from the northwest. We have what is called a "Mediterranean" climate because this rare pattern also occurs in the Mediterranean region of Europe.

Clear skies tend to loose daytime heat, so evenings here would be somewhat cool anyway, but there is even more going on. As the prevailing winds move across the ocean, they reach the edge of the continent, meet the Coast Range mountains, and are forced into an air current that moves along the coast toward the south. This wind pushes water in front of it to form an ocean current, the "California Current", which also moves south along the continent's edge. If the water in this current were the same temperature as the rest of the ocean around our latitude, it would be rather mild. Once again though, because the earth is rotating, the water in the current turns away from the coast, and since water that replaces it can't come from land, it comes from underneath, where temperatures are much colder. This is called "upwelling", and it is for this reason that sea life is so abundant hereÑthe cold, upwelled water in the California Current is full of nutrients that the web of life in the sea thrives on. Prevailing winds blow over this cold current, and they have enough moisture in them near the ocean surface that when they meet the cold current they condense, like your breath does on a cold day, making fog. Meanwhile, daytime heat in the interior valleys rises, drawing in the cooler air off the ocean to fill the space it leaves behind, and, the fog is drawn in with it, as fog or as a low cloud formation.

As days get hotter, fog is drawn further inland, eventually cooling the interior valleys so that there isn't as much warm air rising in them, and the fog stops flowing inland. This happens in a cycle, so that here in our valley, fog seems thicker and persists longer and then is gone for a few days. If we lived in much of the world in summer at this latitude, it would be warm in the evenings, sometimes as hot as it was in the day, and we would no doubt be thinking what it would be like if we lived in Northern California, with grapes and redwoods and other beings that know the value of summer fog.